[5.4] Implement standalone if empty in a NB-Change-Way

[LKaemmerling]

As addition of #18425 it would be nice if the @empty statement can be used outside of an forelese.

This PR implements this in a Non Breaking Change way.

So you can use it with:

@empty($array)
This Array is Empty!
@endempty

instead of

@if(empty($array))
This Array is Empty!
@endif

This has no impact to the @forelse statement.

[LKaemmerling]

Thank you!