fix(graph): prevent SQL crash on empty LLM registry during migration

migrate_llm_models was generating invalid SQL 'NOT IN ()' when the
LLM registry is empty (e.g., fresh deployment before seeding). This
causes a fatal PostgreSQL error and prevents server startup.

Skip the migration with a warning if enum_values is empty.

autogpt_platform/backend/backend/data/graph.py

@@ -1667,6 +1667,12 @@ async def migrate_llm_models(migrate_to: LlmModel):
     from backend.data import llm_registry
 
     enum_values = list(llm_registry.get_all_model_slugs_for_validation())
+    
+    # Skip migration if registry is empty (fresh deployment before seeding)
+    if not enum_values:
+        logger.warning("LLM registry is empty, skipping model migration")
+        return
+    
     escaped_enum_values = repr(tuple(enum_values))  # hack but works
 
     # Update each block