Refactor get_profile: do not return missing fields.

[clokep]

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.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
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• Code style is correct
  (run the linters)

[clokep]

Actually I think this is missing one change to the servlet which makes the changes a bit clearer of why we want them.

[anoadragon453]

Actually I think this is missing one change to the servlet which makes the changes a bit clearer of why we want them.

@clokep Should we mark this as a draft until that's done?

[clokep]

Actually I think this is missing one change to the servlet which makes the changes a bit clearer of why we want them.

@clokep Should we mark this as a draft until that's done?

Whatever works in your process. I won't have time to get to it today.

[clokep]

@anoadragon453 This should be ready to review now.

[anoadragon453]

LGTM! Nice idea to use a constant to differentiate from custom ones. And thanks for splitting this out.