fix(pangea): use elif to prevent UnboundLocalError and handle None messages

Address Greptile review feedback:
- Make branches mutually exclusive using elif to prevent input_messages from being overwritten
- Handle case where data.get('messages') returns None to avoid passing invalid payload to Pangea API

litellm/proxy/guardrails/guardrail_hooks/pangea/pangea.py

@@ -250,10 +250,13 @@ class PangeaHandler(CustomGuardrail):
         if isinstance(response, TextCompletionResponse):
             # Assume the earlier call type as well
             input_messages = _TextCompletionRequest(data).get_messages()
-        if not isinstance(response, ModelResponse):
-            return
+        elif isinstance(response, ModelResponse):
+            messages = data.get("messages")
+            if messages is None:
+                return  # No messages to check
+            input_messages = cast(List[Dict[Any, Any]], messages)
         else:
-            input_messages = cast(List[Dict[Any, Any]], data.get("messages"))
+            return
 
         if choices := response.get("choices"):
             if isinstance(choices, list):