Fix annotation in UtcDateTime#process_result_value

The argument can only be a string or None, because
dateime.from_isoformat expects only a string.

portal/model/UtcDateTime.py

@@ -36,7 +36,7 @@ class UtcDateTime(types.TypeDecorator):
         return None
 
     def process_result_value(
-        self, value: str | datetime, dialect: Dialect
+        self, value: str | None, dialect: Dialect
     ) -> Optional[datetime]:
         if value is None:
             return None