Unhandled exception in FlowAuth backend when adding a user with empty password

[jc-harrison]

Describe the bug
FlowAuth admin add_user fails with the following exception when json["password"] is an empty string:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2446, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1951, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1820, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1949, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1935, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.6/site-packages/flask_login/utils.py", line 261, in decorated_view
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/flask_principal.py", line 199, in _decorated
    rv = f(*args, **kw)
  File "/usr/local/lib/python3.6/site-packages/flowauth/admin.py", line 639, in add_user
    if zxcvbn(json["password"])["score"] > 3:
  File "/usr/local/lib/python3.6/site-packages/zxcvbn/__init__.py", line 28, in zxcvbn
    result = scoring.most_guessable_match_sequence(password, matches)
  File "/usr/local/lib/python3.6/site-packages/zxcvbn/scoring.py", line 204, in most_guessable_match_sequence
    optimal_match_sequence = unwind(n)
  File "/usr/local/lib/python3.6/site-packages/zxcvbn/scoring.py", line 181, in unwind
    for candidate_l, candidate_g in optimal['g'][k].items():
IndexError: list index out of range

Expected behaviour here should be to raise

InvalidUsage(
    "Password not long enough.", payload={"bad_field": "password"}
)

or similar.