fix problem when missing reviewers

[marwin1991]

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/valhalla/__main__.py", line 4, in <module>
    start()
  File "/opt/valhalla/valhalla/main.py", line 35, in start
    create_merge_request(config.merge_request)
  File "/opt/valhalla/valhalla/main.py", line 57, in create_merge_request
    merge_request.create(merge_request_config)
  File "/opt/valhalla/valhalla/ci_provider/gitlab/merge_request.py", line 45, in create
    'reviewer_ids': self.__get_reviewer_ids(merge_request_config.reviewers)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/valhalla/valhalla/ci_provider/gitlab/merge_request.py", line 54, in __get_reviewer_ids
    for rev in reviewers:
TypeError: 'NoneType' object is not iterable