fix variable key can't null or empty

Author: wangliang181230

apps/application/flow/step_node/variable_aggregation_node/i_variable_aggregation_node.py

@@ -11,7 +11,7 @@
 
 class VariableListSerializer(serializers.Serializer):
     v_id = serializers.CharField(required=True, label=_("Variable id"))
-    key = serializers.CharField(required=False, label=_("Key"))
+    key = serializers.CharField(required=False, label=_("Key"), allow_null=True, allow_blank=True, )
     variable = serializers.ListField(required=True, label=_("Variable"))