Fix running the web UI with class defined hosts

[chaen]

I have the use case of two clients, each defining host as a class attribute. When running with --headless, it works fine, but when running with the UI, I kept getting errors like

                                               
[2021-12-09 14:32:00,340] localhost.localdomain/ERROR/locust.user.task: Invalid URL '/index.html': No schema supplied. Perhaps you meant http:///index.html?                    
Traceback (most recent call last):                                                                                                                                                            
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/locust/user/task.py", line 296, in run                                                                 
    self.execute_next_task()
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/locust/user/task.py", line 321, in execute_next_task
    self.execute_task(self._task_queue.pop(0))
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/locust/user/task.py", line 434, in execute_task
    task(self.user)
  File "/home/chaen/dirac/diracScaleTests/bug.py", line 15, in hello_world
    self.client.get("/index.html")
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/requests/sessions.py", line 555, in get
    return self.request('GET', url, **kwargs)
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/locust/clients.py", line 128, in request
    response = self._send_request_safe_mode(method, url, **kwargs)
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/locust/clients.py", line 195, in _send_request_safe_mode
    return super().request(method, url, **kwargs)
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/requests/sessions.py", line 528, in request
    prep = self.prepare_request(req)
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/requests/sessions.py", line 456, in prepare_request
    p.prepare(
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/requests/models.py", line 316, in prepare
    self.prepare_url(url, params)
  File "/home/chaen/miniconda3/envs/diracprod/lib/python3.9/site-packages/requests/models.py", line 390, in prepare_url
    raise MissingSchema(error)
requests.exceptions.MissingSchema: Invalid URL '/index.html': No schema supplied. Perhaps you meant http:///index.html?

This is because the web UI returns '' as host when left empty (as opposed to None that the code expected).

I changed the test conditions for overwriting the values, and added a test

[cyberw]

Thx!