Handle issue events not linked to any issue

[ericboucher]

I am unsure of what situation it reflects, but I have been seeing the following error on issue_events where there doesn't seem to be any linked issue and we get an AttributeError: 'NoneType' object has no attribute 'pop'.

To start, we could simply add a check for the issue object and set null issue_number and issue_url if the object does not exist. Thoughts?

time=2021-09-27 15:42:20 name=tap-github level=INFO message=INFO METRIC: {'type': 'timer', 'metric': 'http_request_duration', 'value': 1.158103, 'tags': {'endpoint': '/repos/{org}/{repo}/issues/events', 'http_status_code': 200, 'status': 'succeeded', 'context': {'org': 'facebook', 'repo': 'react'}}}
Traceback (most recent call last):
.../...
tract/.venv/lib/python3.9/site-packages/tap_github/streams.py", line 529, in post_process
    row["issue_number"] = int(row["issue"].pop("number"))
AttributeError: 'NoneType' object has no attribute 'pop'

[ericboucher]

Issue events can take multiple shapes. In particular, if they only refer to a PR and not an issue, the issue field might not exist.

We should augment the proposed schema and aim towards something a bit more complete, as was done in https://github.com/singer-io/tap-github/blob/master/tap_github/schemas/issue_events.json