[Bugfix] Proper input validation for multi-modal encoder-decoder models (#16156)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

tests/entrypoints/openai/test_prompt_validation.py

@@ -17,7 +17,7 @@ async def test_empty_prompt():
         client = remote_server.get_async_client()
 
         with pytest.raises(openai.BadRequestError,
-                           match=re.compile('.+Prompt cannot be empty.+')):
+                           match="decoder prompt cannot be empty"):
             await client.completions.create(model=model_name,
                                             prompt="",
                                             max_tokens=5,