[5.x] Fix filterWhere with arrays

[aerni]

This PR fixes a Cannot access offset of type array in isset or empty exception that was likely introduced with #12894 or #13266.

I ran into it because of PR #11870. The exception was caused by the following query in 75d76d0:

$query->whereIn('author', [User::current()->id()]);

I've removed this query in favor of the new whereHas() as seen in the commit. This resolves the issue in that particular PR.

I thought it was worth raising the issue anyway and providing a fix for it.