should return invalide request for empty prompt (#9315)

python/sglang/srt/entrypoints/openai/serving_completions.py

 import logging
 import time
-from typing import Any, AsyncGenerator, Dict, List, Union
+from typing import Any, AsyncGenerator, Dict, List, Optional, Union
 
 from fastapi import Request
 from fastapi.responses import ORJSONResponse, StreamingResponse
@@ -41,6 +41,14 @@ class OpenAIServingCompletion(OpenAIServingBase):
     def _request_id_prefix(self) -> str:
         return "cmpl-"
 
+    def _validate_request(self, request: CompletionRequest) -> Optional[str]:
+        """Validate that the input is valid."""
+        prompt = request.prompt
+        if not prompt or (isinstance(prompt, list) and all(not p for p in prompt)):
+            return "Prompt cannot be empty"
+
+        return None
+
     def _convert_to_internal_request(
         self,
         request: CompletionRequest,