perf(duckdb): reduce branching factor for ArrayDistinct

If I do something like `some_really_complex_array_expression.unique()`, then the SQL for `some_really_complex_array_expression` was duplicated 4 times. Now it is only duplicated 3 times.

This is important for perf, because duckdb sometimes appears to not be smart, and actually does a computation for each time a subexpression appears.
See duckdb/duckdb#14649.
So this can reduce the computation time to 3/4 of what it was.

I want to go through our other compilation steps and do similar optimizations whenever possible.

ibis/backends/sql/compilers/duckdb.py

@@ -143,15 +143,17 @@ def visit_StructColumn(self, op, *, names, values):
         )
 
     def visit_ArrayDistinct(self, op, *, arg):
+        return self._array_distinct(arg)
+
+    def _array_distinct(self, arg: sge.Expression) -> sge.Expression:
+        x = sg.to_identifier("x")
+        is_null = sge.Lambda(this=x.is_(NULL), expressions=[x])
+        contains_null = self.f.list_bool_or(self.f.list_apply(arg, is_null))
         return self.if_(
             arg.is_(NULL),
             NULL,
             self.f.list_distinct(arg)
-            + self.if_(
-                self.f.list_count(arg) < self.f.len(arg),
-                self.f.array(NULL),
-                self.f.array(),
-            ),
+            + self.if_(contains_null, self.f.array(NULL), self.f.array()),
         )
 
     def visit_ArrayPosition(self, op, *, arg, other):
@@ -223,16 +225,7 @@ def visit_ArrayRemove(self, op, *, arg, other):
 
     def visit_ArrayUnion(self, op, *, left, right):
         arg = self.f.list_concat(left, right)
-        return self.if_(
-            arg.is_(NULL),
-            NULL,
-            self.f.list_distinct(arg)
-            + self.if_(
-                self.f.list_count(arg) < self.f.len(arg),
-                self.f.array(NULL),
-                self.f.array(),
-            ),
-        )
+        return self._array_distinct(arg)
 
     def visit_ArrayZip(self, op, *, arg):
         i = sg.to_identifier("i")

ibis/backends/tests/test_array.py

@@ -779,8 +779,8 @@ def test_array_remove(con, input, expected):
     ("input", "expected"),
     [
         param(
-            {"a": [[1, 3, 3], [], [42, 42], [], [None], None]},
-            [{3, 1}, set(), {42}, set(), {None}, None],
+            {"a": [[1, 3, 3], [1, 3, None, 3], [42, 42], [], [None], None]},
+            [{3, 1}, {1, 3, None}, {42}, set(), {None}, None],
             id="null",
             marks=[
                 pytest.mark.notyet(