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

This reverts commit 4cb82f0e7d02ec268021200f88f970ffc8b07d5f.
Throw an execption in `Model::__callStatic()` if the method is unknown (see #6160)
Fix endless recursion in `loadDataContainer()` (see #6145)
Fix two issues in the `Database_Result` class
Do not exit after a template has been output (see #5570)
Apply the width and height to the audio player as well (see #6114)
Replace insert tags in the details view of the listing module (see #6120)
Support `tel:` and `sms:` upon IDNA conversion (see #6148)
Unset the primary key if a model is deleted (see #6162)
Check whether a file exists when adding enclosures (see #6173)
Do not cache the "toggle_view" insert tag (see #6172)
Strip the related fields before passing a DB result to `Model::setRow()` (see #6133)
Do not throw an Exception if a model does not (yet) reference a relation (see #6161)
Use `display:table` to clear floats in the back end (see #6093)
Use absolute URLs in the front end preview redirect (see #4332)
Hide the `lastLogin` field in the details view (see #5949)
Make the CSS ID available in the custom navigation module (see #6129)
Correctly link to news and calendar feeds via insert tag (see #6164)
Do not require a `DataContainer` object in `tl_content::showJsLibraryHint()` (see #6157)
Only show the "build the internal cache" message to admins (see #6156)