[BugFix] Fix server crash on empty prompt (#7746)

Signed-off-by: Max de Bayser <mbayser@br.ibm.com>

tests/entrypoints/llm/test_prompt_validation.py

@@ -0,0 +1,9 @@
+import pytest
+
+from vllm import LLM
+
+
+def test_empty_prompt():
+    llm = LLM(model="gpt2")
+    with pytest.raises(ValueError, match='Prompt cannot be empty'):
+        llm.generate([""])