Fix optional parameter parsing in MiniMax M2 tool parser #32278 (#32342)

Signed-off-by: baonudesifeizhai <baonudesifeizhai@gmail.com>

vllm/tool_parsers/minimax_m2_tool_parser.py

@@ -217,16 +217,13 @@ class MinimaxM2ToolParser(ToolParser):
         Returns:
             The converted value
         """
-        if value.lower() == "null":
+        # Check if the VALUE itself indicates null (not just if null is allowed)
+        if value.lower() in ("null", "none", "nil"):
             return None
 
         # Normalize types
         normalized_types = [t.lower() for t in param_types]
 
-        # Try null first if it's in the list
-        if "null" in normalized_types or value.lower() in ("null", "none", "nil"):
-            return None
-
         # Try each type in order of preference (most specific first, string as fallback)
         # Priority: integer > number > boolean > object > array > string
         type_priority = [