api: cannot split repos when it is NULL
    
    Adressing:
    
    ERROR:coprs.error_handlers:Admin-only exception
    Request: GET /api_3/project/list?limit=1000
    User: msuchy
    Traceback (most recent call last):
      File "/usr/lib/python3.10/site-packages/flask/app.py", line 1513, in full_dispatch_request
        rv = self.dispatch_request()
      File "/usr/lib/python3.10/site-packages/flask/app.py", line 1499, in dispatch_request
        return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
      File "/usr/share/copr/coprs_frontend/coprs/views/apiv3_ns/__init__.py", line 71, in pagination_wrapper
        return f(*args, **kwargs)
      File "/usr/share/copr/coprs_frontend/coprs/views/apiv3_ns/__init__.py", line 58, in query_params_wrapper
        return f(*args, **kwargs)
      File "/usr/share/copr/coprs_frontend/coprs/views/apiv3_ns/apiv3_projects.py", line 106, in get_project_list
        projects = paginator.map(to_dict)
      File "/usr/share/copr/coprs_frontend/coprs/views/apiv3_ns/__init__.py", line 157, in map
        return [fun(x) for x in self.get()]
      File "/usr/share/copr/coprs_frontend/coprs/views/apiv3_ns/__init__.py", line 157, in <listcomp>
        return [fun(x) for x in self.get()]
      File "/usr/share/copr/coprs_frontend/coprs/views/apiv3_ns/apiv3_projects.py", line 34, in to_dict
        "additional_repos": copr.repos_list,
      File "/usr/share/copr/coprs_frontend/coprs/models.py", line 433, in repos_list
        return self.repos.split()
    AttributeError: 'NoneType' object has no attribute 'split'