Refactor get_profile: do not return missing fields. (#18063)

Refactor `get_profile` to avoid returning "empty" (`None` / `null`)
fields. Currently this is not very important, but will be more useful
once #17488 lands. It does update the servlet to use this now which has
a minor change in behavior: additional fields served over federation
will now be properly sent back to clients.

It also adds constants for `avatar_url` / `displayname` although I did
not attempt to use it everywhere possible.