error when calling generate_authorize_redirect about authlib

[lepture]

Actually, it is caused by uri=None:

>>> add_params_to_uri(None, [('client_id', None)])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/lepture/workspace/lepture/authlib/authlib/common/urls.py", line 117, in add_params_to_uri
    return urlparse.urlunparse((sch, net, path, par, query, fra))
  File "/Users/lepture/.pyenv/versions/3.6.2/lib/python3.6/urllib/parse.py", line 454, in urlunparse
    _coerce_args(*components))
  File "/Users/lepture/.pyenv/versions/3.6.2/lib/python3.6/urllib/parse.py", line 120, in _coerce_args
    raise TypeError("Cannot mix str and non-str arguments")
TypeError: Cannot mix str and non-str arguments
>>>

[lepture]

@timfeirg I've added a validation for authorize_url. close it for now.