Follow-up of #1212: set value to 'None' when unset

[leplatrem]

I was able to reproduce the error locally:

>>> s.client.update_issue_field(key="RMST-212", fields={"priority":"None"})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
 ...
  File "/Users/mathieu/Code/Mozilla/jira-bugzilla-integration/jbi/jira/client.py", line 53, in raise_for_status
    return super().raise_for_status(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mathieu/Code/Mozilla/jira-bugzilla-integration/.venv/lib/python3.12/site-packages/atlassian/rest_client.py", line 999, in raise_for_status
    raise HTTPError(error_msg, response=response)
requests.exceptions.HTTPError: PUT /rest/api/2/issue/RMST-212?notifyUsers=true -> HTTP 400: Specify the Priority (id or name) in the string format

>>> s.client.update_issue_field(key="RMST-212", fields={"priority":{"name": "None"}})
>>> s.client.update_issue_field(key="RMST-212", fields={"priority":{"name": "P1"}})
>>>