🐛 Source Zendesk Support: fix tickets_audits stream schema

[bazarnov]

What

https://github.com/airbytehq/airbyte/runs/6488173868?check_suite_focus=true#step:11:3085

----------------------------- Captured stdout call -----------------------------
***"type": "LOG", "log": ***"level": "ERROR", "message": "\nThe ticket_audits stream has the following schema errors:\n45 is not of type 'null', 'string'\n\nFailed validating 'type' in schema['properties']['events']['items']['properties']['data']['properties']['call_duration']:\n    ***'type': ['null', 'string']***\n\nOn instance['events'][0]['data']['call_duration']:\n    45"***
------------------------------ Captured log call -------------------------------
ERROR    root:test_core.py:300 
The ticket_audits stream has the following schema errors:
45 is not of type 'null', 'string'

Failed validating 'type' in schema['properties']['events']['items']['properties']['data']['properties']['call_duration']:
    ***'type': ['null', 'string']***

On instance['events'][0]['data']['call_duration']:
    45
=========================== short test summary info ============================
FAILED test_core.py::TestBasicRead::test_read[inputs0] - Failed: Please check...
=================== 1 failed, 29 passed in 175.28s (0:02:55) ===================