Add test to cover missing line in serialize_sequence_value

Updated test_serialize_sequence_value_with_none_first_in_union to use Union[None, list[str]]
instead of list[str] | None to ensure None appears first in union args, which properly tests
the continue statement at line 316 in fastapi/_compat/v2.py.

Co-authored-by: tiangolo <[email protected]>

tests/test_compat.py

@@ -121,8 +121,10 @@ def test_serialize_sequence_value_with_optional_list_pipe_union():
 def test_serialize_sequence_value_with_none_first_in_union():
     """Test that serialize_sequence_value handles Union[None, List[...]] correctly."""
     from fastapi._compat import v2
+    from typing import Union
 
-    field_info = FieldInfo(annotation=list[str] | None)
+    # Use Union[None, list[str]] to ensure None comes first in the union args
+    field_info = FieldInfo(annotation=Union[None, list[str]])
     field = v2.ModelField(name="items", field_info=field_info)
     result = v2.serialize_sequence_value(field=field, value=["x", "y"])
     assert result == ["x", "y"]