Fix an issue where we would send an invalid /user/keys/query request

[S7evinK]

Discovered while checking Grafana and wondering why there was one server being hit over and over again.
Turns out that we were sending

{
  "device_keys": {
    "@alice:example.com": null
  }
}

instead of an empty list.

This should fix it, even though the server in question still seems to return an invalid response.

Pull Request Checklist

• Pull request includes a sign off

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.55%. Comparing base (c70b213) to head (28a1845).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
+ Coverage   59.43%   59.55%   +0.12%     
==========================================
  Files          51       51              
  Lines        7257     7262       +5     
==========================================
+ Hits         4313     4325      +12     
+ Misses       2544     2536       -8     
- Partials      400      401       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[S7evinK]

As the commit message suggests, tests are generated by Copilot.