[Bugfix] Do not add extra \n for image-only cases when constructing multimodal text prompts. (#33647)

Signed-off-by: wang.yuqi <yuqi.wang@daocloud.io>

vllm/entrypoints/chat_utils.py

@@ -1164,7 +1164,10 @@ def _get_full_multimodal_text_prompt(
 
     # NOTE: Default behaviour: we always add missing placeholders
     # at the front of the prompt, if interleave_strings=False
-    return "\n".join(missing_placeholders + [text_prompt])
+    if text_prompt:
+        return "\n".join(missing_placeholders + [text_prompt])
+    else:
+        return "\n".join(missing_placeholders)
 
 
 # No need to validate using Pydantic again