fix: The discriminator is empty and the judgment is incorrect (#4166)

Author: shaohuzhang1

apps/application/flow/compare/is_null_compare.py

@@ -18,4 +18,7 @@ def support(self, node_id, fields: List[str], source_value, compare, target_valu
             return True
 
     def compare(self, source_value, compare, target_value):
-        return source_value is None or len(source_value) == 0
+        try:
+            return source_value is None or len(source_value) == 0
+        except Exception as e:
+            return False