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