Skip empty lines when determining base indentation (#9795)

## Summary

It turns out we saw a panic in cases when dedenting blocks like the `def
wrapper` here:

```python
def instrument_url(f: UrlFuncT) -> UrlFuncT:
    # TODO: Type this with ParamSpec to preserve the function signature.
    if not INSTRUMENTING:  # nocoverage -- option is always enabled; should we remove?
        return f
    else:

        def wrapper(
            self: "ZulipTestCase", url: str, info: object = {}, **kwargs: Union[bool, str]
        ) -> HttpResponseBase:
```

Since we relied on the first line to determine the indentation, instead
of the first non-empty line.

## Test Plan

`cargo test`

crates/ruff_linter/resources/test/fixtures/flake8_return/RET505.py

@@ -198,3 +198,43 @@ def bar9():
 def sb(self):
     if self._sb is not None: return self._sb
     else: self._sb = '\033[01;%dm'; self._sa = '\033[0;0m';
+
+
+def indent(x, y, w, z):
+    if x:  # [no-else-return]
+        a = 1
+        return y
+    else:
+
+        c = 3
+        return z
+
+
+def indent(x, y, w, z):
+    if x:  # [no-else-return]
+        a = 1
+        return y
+    else:
+        # comment
+        c = 3
+        return z
+
+
+def indent(x, y, w, z):
+    if x:  # [no-else-return]
+        a = 1
+        return y
+    else:
+          # comment
+        c = 3
+        return z
+
+
+def indent(x, y, w, z):
+    if x:  # [no-else-return]
+        a = 1
+        return y
+    else:
+  # comment
+        c = 3
+        return z

crates/ruff_linter/src/rules/flake8_return/snapshots/ruff_linter__rules__flake8_return__tests__RET505_RET505.py.snap

@@ -171,4 +171,48 @@ RET505.py:200:5: RET505 Unnecessary `else` after `return` statement
     |
     = help: Remove unnecessary `else`
 
+RET505.py:207:5: RET505 Unnecessary `else` after `return` statement
+    |
+205 |         a = 1
+206 |         return y
+207 |     else:
+    |     ^^^^ RET505
+208 | 
+209 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
+RET505.py:217:5: RET505 Unnecessary `else` after `return` statement
+    |
+215 |         a = 1
+216 |         return y
+217 |     else:
+    |     ^^^^ RET505
+218 |         # comment
+219 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
+RET505.py:227:5: RET505 Unnecessary `else` after `return` statement
+    |
+225 |         a = 1
+226 |         return y
+227 |     else:
+    |     ^^^^ RET505
+228 |           # comment
+229 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
+RET505.py:237:5: RET505 Unnecessary `else` after `return` statement
+    |
+235 |         a = 1
+236 |         return y
+237 |     else:
+    |     ^^^^ RET505
+238 |   # comment
+239 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
 

crates/ruff_linter/src/rules/flake8_return/snapshots/ruff_linter__rules__flake8_return__tests__preview__RET505_RET505.py.snap

@@ -358,5 +358,107 @@ RET505.py:200:5: RET505 [*] Unnecessary `else` after `return` statement
 199 199 |     if self._sb is not None: return self._sb
 200     |-    else: self._sb = '\033[01;%dm'; self._sa = '\033[0;0m';
     200 |+    self._sb = '\033[01;%dm'; self._sa = '\033[0;0m';
+201 201 | 
+202 202 | 
+203 203 | def indent(x, y, w, z):
+
+RET505.py:207:5: RET505 [*] Unnecessary `else` after `return` statement
+    |
+205 |         a = 1
+206 |         return y
+207 |     else:
+    |     ^^^^ RET505
+208 | 
+209 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
+ℹ Safe fix
+204 204 |     if x:  # [no-else-return]
+205 205 |         a = 1
+206 206 |         return y
+207     |-    else:
+208 207 | 
+209     |-        c = 3
+210     |-        return z
+    208 |+    c = 3
+    209 |+    return z
+211 210 | 
+212 211 | 
+213 212 | def indent(x, y, w, z):
+
+RET505.py:217:5: RET505 [*] Unnecessary `else` after `return` statement
+    |
+215 |         a = 1
+216 |         return y
+217 |     else:
+    |     ^^^^ RET505
+218 |         # comment
+219 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
+ℹ Safe fix
+214 214 |     if x:  # [no-else-return]
+215 215 |         a = 1
+216 216 |         return y
+217     |-    else:
+218     |-        # comment
+219     |-        c = 3
+220     |-        return z
+    217 |+    # comment
+    218 |+    c = 3
+    219 |+    return z
+221 220 | 
+222 221 | 
+223 222 | def indent(x, y, w, z):
+
+RET505.py:227:5: RET505 [*] Unnecessary `else` after `return` statement
+    |
+225 |         a = 1
+226 |         return y
+227 |     else:
+    |     ^^^^ RET505
+228 |           # comment
+229 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
+ℹ Safe fix
+224 224 |     if x:  # [no-else-return]
+225 225 |         a = 1
+226 226 |         return y
+227     |-    else:
+228     |-          # comment
+229     |-        c = 3
+230     |-        return z
+    227 |+      # comment
+    228 |+    c = 3
+    229 |+    return z
+231 230 | 
+232 231 | 
+233 232 | def indent(x, y, w, z):
+
+RET505.py:237:5: RET505 [*] Unnecessary `else` after `return` statement
+    |
+235 |         a = 1
+236 |         return y
+237 |     else:
+    |     ^^^^ RET505
+238 |   # comment
+239 |         c = 3
+    |
+    = help: Remove unnecessary `else`
+
+ℹ Safe fix
+234 234 |     if x:  # [no-else-return]
+235 235 |         a = 1
+236 236 |         return y
+237     |-    else:
+238 237 |   # comment
+239     |-        c = 3
+240     |-        return z
+    238 |+    c = 3
+    239 |+    return z
 
 

crates/ruff_python_trivia/src/textwrap.rs

@@ -138,11 +138,18 @@ pub fn dedent(text: &str) -> Cow<'_, str> {
 /// # Panics
 /// If the first line is indented by less than the provided indent.
 pub fn dedent_to(text: &str, indent: &str) -> String {
-    // Look at the indentation of the first line, to determine the "baseline" indentation.
+    // Look at the indentation of the first non-empty line, to determine the "baseline" indentation.
     let existing_indent_len = text
         .universal_newlines()
-        .next()
-        .map_or(0, |line| line.len() - line.trim_start().len());
+        .find_map(|line| {
+            let trimmed = line.trim_whitespace_start();
+            if trimmed.is_empty() || trimmed.starts_with('#') {
+                None
+            } else {
+                Some(line.len() - trimmed.len())
+            }
+        })
+        .unwrap_or_default();
 
     // Determine the amount of indentation to remove.
     let dedent_len = existing_indent_len - indent.len();