Some users were complaining that when the user unique IP limiter was
enabled, they would be banned for some unknown
reason. `AuthFinder.find_for_git_client` can authenticate users from a
multitude of tokens (CI, LFS, HTTP basic auth, etc.), but project deploy
tokens are unique in that they aren't attributed to a specific user. As
a result, if project deploy tokens were used, users that had the same
database ID as a deploy token would erroneously be attributed to using
the IP accessed by the token.

To fix this issue, we only call `Gitlab::Auth::UniqueIpsLimiter` if a
user is returned from the authentication search. Project deploy tokens
could be used from many different IPs, so it doesn't make sense to group
them with user activity.

Possibly fixes https://gitlab.com/gitlab-org/gitlab/issues/22854