fix(serializer): NaN to string rather than None

Author: hassiebp

langfuse/_utils/serializer.py

@@ -63,7 +63,7 @@ def default(self, obj: Any):
                 return obj.tolist()
 
             if isinstance(obj, float) and math.isnan(obj):
-                return None
+                return "NaN"
 
             if isinstance(obj, float) and math.isinf(obj):
                 return "Infinity"