fix: set empty tool_call_parser to None

src/engine_args.py

@@ -91,7 +91,7 @@ DEFAULT_ARGS = {
     "otlp_traces_endpoint": os.getenv('OTLP_TRACES_ENDPOINT', None),
     "use_v2_block_manager": os.getenv('USE_V2_BLOCK_MANAGER', 'true'),
     "enable_auto_tool_choice": os.getenv('ENABLE_AUTO_TOOL_CHOICE', 'false').lower() == 'true',
-    "tool_call_parser": os.getenv('TOOL_CALL_PARSER', None)
+    "tool_call_parser": os.getenv('TOOL_CALL_PARSER', "") or None
 }
 
 def match_vllm_args(args):