500 on Kippo Github project create action when no project-manager is defined for KippoProject

[monkut]

An exception is raised when a GithubProject for a KippoProject is attempted to be created via the KippoProject' admin action.

  File "/var/task/django/contrib/admin/options.py", line 1700, in changelist_view
  response = self.response_action(request, queryset=cl.get_queryset(request))
  File "/var/task/django/contrib/admin/options.py", line 1400, in response_action
  response = func(self, request, queryset)
  File "/var/task/projects/admin.py", line 134, in create_github_organizational_project_action
  description=kippo_project.github_project_description,
  File "/var/task/projects/models.py", line 242, in github_project_description
  description = (f"""project_manager: {self.project_manager.display_name}<br/>"""
AttributeError: 'NoneType' object has no attribute 'display_name'

Update to show error to user if project manager is not defined, or ignore project manager input/display if not given.