Resistance variants page crashes on MRSA samples

[mhkc]

Describe the bug The resistance variants view crashes with a 500 error.

To Reproduce

• MT23000029

Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/flask/app.py", line 2190, in wsgi_app
    response = self.full_dispatch_request()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/flask/app.py", line 1486, in full_dispatch_request
    rv = self.handle_user_exception(e)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/flask/app.py", line 1484, in full_dispatch_request
    rv = self.dispatch_request()
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/flask/app.py", line 1469, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/flask_login/utils.py", line 290, in decorated_view
    return current_app.ensure_sync(func)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/worker/app/app/blueprints/sample/views.py", line 259, in resistance_variants
    sample_info = sort_variants(sample_info)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/worker/app/app/blueprints/sample/controllers.py", line 201, in sort_variants
    sample_info[variant_type] = sorted(sample_info[variant_type], key=_sort_func)
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: 'NoneType' object is not iterable