TextgridConfig does not check for `None` on `__init__`

type hint states Optional[str] for __init__; this inferes that None could be used as an argument but this is not checked against. With a None value provided for host, __init__ errors with

...tgclients/config.py", line 20, in __init__
    if host.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'

another simple solution could also be to drop Optional in this case