Fix get_asset_data method to handle invalid objref

Author: ajhai

llmstack/assets/apis.py

@@ -40,6 +40,9 @@ def get_by_objref(self, request, objref, include_data=False, include_name=False)
         return DRFResponse(asset)
 
     def get_asset_data(self, objref, request_user, request_session=None, include_data=False, include_name=False):
+        if objref is None or not objref.startswith("objref://"):
+            return None
+
         try:
             category, asset_uuid = objref.split("objref://")[1].split("/")