refactor: Update langflow schema field module to use consistent type hinting for better clarity and consistency

src/backend/base/langflow/template/field/base.py

@@ -1,5 +1,13 @@
 from enum import Enum
-from typing import Any, Callable, GenericAlias, Optional, Union, _GenericAlias, _UnionGenericAlias  # type: ignore
+from typing import (
+    Any,
+    Callable,
+    GenericAlias,
+    Optional,  # type: ignore
+    Union,
+    _GenericAlias,
+    _UnionGenericAlias,
+)
 
 from pydantic import BaseModel, ConfigDict, Field, field_serializer, field_validator, model_serializer, model_validator
 
@@ -181,6 +189,8 @@ class Output(BaseModel):
 
     def add_types(self, _type: list[Any]):
         for type_ in _type:
+            if self.types is None:
+                self.types = []
             self.types.append(type_)
 
     def set_selected(self):