Fix 'completion' referenced before assignment (#800)

- Fixes `UnboundLocalError("local variable 'completion' referenced before assignment")` error in LangchainTracer.on_llm_end callback

backend/chainlit/langchain/callbacks.py

@@ -534,6 +534,7 @@ def _on_run_update(self, run: Run) -> None:
 
                 current_step.language = "json"
                 current_step.output = json.dumps(message_completion)
+                completion = message_completion.get("content", "")
             else:
                 completion_start = self.completion_generations[str(run.id)]
                 completion = generation.get("text", "")