Change UserByIPResource to return 200 instead of 404 on nonexistent user

[lukasjuhrich]

Since sentry (rightfully) interprets 4xx status codes as failures, we should not emit one if sipa requests a user to an IP which does not exist (which is part of normal operation). Instead, pycroft should return an empty result.

The behavior has to be adapted both in pycroft and in sipa.