Fix wrong type on enum_as_str by removing Optional

Author: philogicae

src/aleph/sdk/utils.py

@@ -115,14 +115,14 @@ async def copy_async_readable_to_buffer(
         buffer.write(chunk)
 
 
-def enum_as_str(obj: Optional[Union[str, Enum]]) -> Optional[str]:
+def enum_as_str(obj: Optional[Union[str, Enum]]) -> str:
     """Returns the value of an Enum, or the string itself when passing a string.
 
     Python 3.11 adds a new formatting of string enums.
     `str(MyEnum.value)` becomes `MyEnum.value` instead of `value`.
     """
     if obj is None:
-        return None
+        return ""
     if not isinstance(obj, str):
         raise TypeError(f"Unsupported enum type: {type(obj)}")