fix: ensure PR body is not None

Description

If the PR body is None, we cannot parse it.

Supporting information

The error:

Traceback (most recent call last):
  File "/home/app/function/handler.py", line 329, in handle
    handler(body)
  File "/home/app/function/handler.py", line 291, in handle_cron_invocation
    list(executor.map(worker, open_pull_requests + closed_pull_requests))
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 619, in result_iterator
    yield _result_or_cancel(fs.pop())
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 317, in _result_or_cancel
    return fut.result(timeout)
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 456, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/usr/local/lib/python3.12/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/app/function/handler.py", line 228, in __handle_state_change
    return gitlab_client.trigger_instance_create_or_update_pipeline(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/app/function/gitlab.py", line 270, in trigger_instance_create_or_update_pipeline
    "applied_settings": params.pull_request.extra_settings,
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/app/function/github.py", line 71, in extra_settings
    return self.__extract_from_body(r"[Ss]ettings", self.body) or ""
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/app/function/github.py", line 120, in __extract_from_body
    if match := re.search(pattern, body):
                ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/re/__init__.py", line 177, in search
    return _compile(pattern, flags).search(string)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected string or bytes-like object, got 'NoneType'

Testing instructions

Steps to test the changes:

1. Deploy the changes to the axim sandbox cluster
2. Trigger a new run
3. Ensure this PR is picked up

Dependencies

NA

Screenshots

NA

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated

Additional context

NA