[Frontend][2/n] remove empty content from _parse_tool_calls_from_content (#28331)

Signed-off-by: Andrew Xia <axia@fb.com>
Co-authored-by: Andrew Xia <axia@fb.com>

vllm/entrypoints/openai/serving_engine.py

@@ -1375,6 +1375,8 @@ class OpenAIServing:
                     for tool_call in tool_call_info.tool_calls
                 )
                 content = tool_call_info.content
+                if content and content.strip() == "":
+                    content = None
             else:
                 # No tool calls.
                 return None, content