Fix thrift/test/py:json_generator_test
Fix thrift/test/py:json_generator_test

Summary:
This diff fixes `thrift/test/py:json_generator_test` failure:

```
  File "JSONGenerateTest.py", line 79, in testGen
AssertionError: False is not true
```

Reviewed By: spalamarchuk

Differential Revision: D13197255

fbshipit-source-id: b872a9a6b2a53118b8da7ddeef083f8a3f61c834

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift