Fix unstable format involving backslash + whitespace at beginning of file

black.py

@@ -736,8 +736,11 @@ def format_str(src_contents: str, *, mode: FileMode) -> FileContent:
         for _ in range(after):
             dst_contents.append(str(empty_line))
         before, after = elt.maybe_empty_lines(current_line)
-        for _ in range(before):
-            dst_contents.append(str(empty_line))
+        # Black should not insert empty lines at the beginning
+        # of the file
+        if len(dst_contents) > 0:
+            for _ in range(before):
+                dst_contents.append(str(empty_line))
         for line in split_line(
             current_line, line_length=mode.line_length, features=split_line_features
         ):

tests/data/beginning_backslash.py

@@ -0,0 +1,12 @@
+\
+
+
+
+
+
+print("hello, world")
+
+# output
+
+
+print("hello, world")

tests/test_black.py

@@ -639,6 +639,14 @@ def test_tuple_assign(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_beginning_backslash(self) -> None:
+        source, expected = read_data("beginning_backslash")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     def test_tab_comment_indentation(self) -> None:
         contents_tab = "if 1:\n\tif 2:\n\t\tpass\n\t# comment\n\tpass\n"
         contents_spc = "if 1:\n    if 2:\n        pass\n    # comment\n    pass\n"