Storage Analytics: Competitions and Users usage not loading

Hello,

After the fix of #1451 (closed), the Usage history of codabench.org starting working again. However, when going to the Competitions or Users Usage tabs, we get an error saying : Could not load storage analytics data I found some logs in the Django container (nothing in site worker):

Internal Server Error: /api/analytics/users_usage/
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.9/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python3.9/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python3.9/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/views.py", line 495, in dispatch
    response = self.handle_exception(exc)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/views.py", line 455, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/views.py", line 492, in dispatch
    response = handler(request, *args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/decorators.py", line 54, in handler
    return func(*args, **kwargs)
  File "/app/src/apps/api/views/analytics.py", line 277, in users_usage
    'name': su['user__username'] + " (" + su['user__email'] + ")",
TypeError: can only concatenate str (not "NoneType") to str
Internal Server Error: /api/analytics/users_usage/
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.9/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python3.9/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python3.9/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/views.py", line 495, in dispatch
    response = self.handle_exception(exc)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/views.py", line 455, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/views.py", line 492, in dispatch
    response = handler(request, *args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/rest_framework/decorators.py", line 54, in handler
    return func(*args, **kwargs)
  File "/app/src/apps/api/views/analytics.py", line 277, in users_usage
    'name': su['user__username'] + " (" + su['user__email'] + ")",
TypeError: can only concatenate str (not "NoneType") to str

This error appears when trying to load the Competitions Usage or Users Usage tabs.