Fix LSP error in UtcDateTime#process_bind_param

According to mypy the previous annotation violated the LSP.
We now use the types of the supertype method and return a
datetime object only if the argument was a datetime-object.

portal/model/UtcDateTime.py

@@ -28,12 +28,12 @@ class UtcDateTime(types.TypeDecorator):
         return types.String(32)
 
     def process_bind_param(
-        self, value: datetime, dialect: Dialect
+        self, value: Any | None, dialect: Dialect
     ) -> Optional[str | datetime]:
-        if value is None:
-            return None
+        if isinstance(value, datetime):
+            return value.astimezone(timezone.utc).isoformat()
 
-        return value.astimezone(timezone.utc).isoformat()
+        return None
 
     def process_result_value(
         self, value: str | datetime, dialect: Dialect