Add tests to `test_compat.py`

tests/test_compat.py

@@ -136,6 +136,30 @@ def test_is_uploadfile_sequence_annotation():
     assert is_uploadfile_sequence_annotation(Union[List[str], List[UploadFile]])
 
 
+@needs_pydanticv2
+def test_serialize_sequence_value_with_optional_list():
+    """Test that serialize_sequence_value handles optional lists correctly."""
+    from fastapi._compat import v2
+
+    field_info = FieldInfo(annotation=Union[List[str], None])
+    field = v2.ModelField(name="items", field_info=field_info)
+    result = v2.serialize_sequence_value(field=field, value=["a", "b", "c"])
+    assert result == ["a", "b", "c"]
+    assert isinstance(result, list)
+
+
+@needs_pydanticv2
+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
+
+    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"]
+    assert isinstance(result, list)
+
+
 @needs_py_lt_314
 def test_is_pv1_scalar_field():
     from fastapi._compat import v1