Fix check_auth to return `None` on failure, not `False`

[reivilibre]

See https://matrix-org.github.io/synapse/latest/password_auth_providers.html#optional-methods

Also note that the source code of Synapse suggests the return value of False triggers an error.

[reivilibre]

I'm having trouble unwrapping the many layers of code we have for these

I have to say I do relate to this statement. I've never seen a more confusing module system, though maybe I'm just too used to seeing this kind of thing in Java. :confounded:

Related lines:

• https://github.com/matrix-org/synapse/blob/e5cdb9e2339e321e8a77a898d362d7fbc476303b/synapse/handlers/auth.py#L2034 (key)
• https://github.com/matrix-org/synapse/blob/e5cdb9e2339e321e8a77a898d362d7fbc476303b/synapse/handlers/auth.py#L2171 (site where the error triggers: if the return value is not None and it's not a tuple, an error is generated)

Out of interest, check_3pid_auth has the same set-up, but the LDAP3 module is set up properly.