text: Title: Automatic token refresh when using client_credentials
  Body: I'm experimenting with the Httpx Client using `client_credentials` grant. The automatic token refresh does not work as I expected.
  
  `client_id` and `client_secret` and `token_endpoint` are given when the client is created, so all necessry information is available to fetch the token. 
  
  When making a request I get a `MissingTokenError` exception because I didn't supply a token:
  https://github.com/lepture/authlib/blob/ee4337cf7c825349dd23870822a3cc7df123097f/authlib/integrations/httpx_client/oauth2_client.py#L198-L202
  
  I had expected that the token would be automatically fetched if none is available and it can automatically be fetched. Thats what the call to `ensure_active_token()` does.
  
  I could cheat the system by specifying a dummy token when creating the client (-1 because of #530):
  ```
  token={'expires_at': -1, 'access_token': ''}
  ```
  
  Is it deliberate that in this situation, when the token can be fetched without user intervention, that an initial token must be supplied (via token keyword or a explicit call to `fetch_token()`)?