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

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

tests/entrypoints/llm/test_prompt_validation.py

@@ -15,7 +15,7 @@ def v1(run_with_both_engines):
 
 def test_empty_prompt():
     llm = LLM(model="openai-community/gpt2", enforce_eager=True)
-    with pytest.raises(ValueError, match='Prompt cannot be empty'):
+    with pytest.raises(ValueError, match='decoder prompt cannot be empty'):
         llm.generate([""])