Revert "Return an empty model collection instead of `null` (see #6147)"

This reverts commit 4cb82f0e7d02ec268021200f88f970ffc8b07d5f.
Do not exit after a template has been output (see #5570)
Apply the width and height to the audio player as well (see #6114)
Support `tel:` and `sms:` upon IDNA conversion (see #6148)
Unset the primary key if a model is deleted (see #6162)
Do not cache the "toggle_view" insert tag (see #6172)
Make the CSS ID available in the custom navigation module (see #6129)
Correctly link to news and calendar feeds via insert tag (see #6164)
Correctly update the RSS feeds if a news item or event changes (see #6102)
Prevent article aliases from using reserved names (see #6066)
Consider the additional arguments in `Frontend::jumpToOrReload()` (see #5734)
Do not redirect to protected pages after logout (see #6210)
Bump the version number
Handle all possible errors when uploading files (see #5934)
Update the `Database_Result` object when updating a model (see #6070)
Fix the version 3.1 changelog
Drop the database query cache (see #6070)
Return an empty model collection instead of `null` (see #6147)
Restore the `User::getData()` method (see #6146)
Revert "Return an empty model collection instead of `null` (see #6147)"

This reverts commit 4cb82f0e7d02ec268021200f88f970ffc8b07d5f.