Implementation of user search provider is empty

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Is your feature request related to a problem? Please describe.

I would like to leverage the search providers in a search application (https://apps.nextcloud.com/apps/thesearchpage).

In that process, I'm looking into the different implementation of Nextcloud OoB providers and some that are declared by third-party application. The implementation of UserSearch is empty. Currently it only returns an empty array.

Describe the solution you'd like

I would like to be able to search for user through the search API.

Describe alternatives you've considered

There is none that I can think of except ignoring that provider.

Start by locating the empty UserSearch implementation and reviewing the other Nextcloud OoB search providers mentioned in the issue. Trace the search API contract and determine how users should be returned; done means UserSearch performs user searches instead of always returning an empty array, with the relevant behavior covered by tests if the project provides them.