[10.x] fix handle shift() on an empty collection

[Treggats]

fixes #51840

This PR fixes how the Collection::shift() method handles an empty collection.
Currently it will return the collection when trying to call shift() on an empty collection. Before it would return null.

steps to reproduce

This is also covered in the added test.

1. create a collection with two objects
2. call shift() three times on it and assign it to variables
3. dump a property of the object for every variable
4. it will throw an exception saying it can not call the property on a collection

[Treggats]

I think this is the correct placement for the isEmpty() check. We want to make sure that there are items to use before we continue to handle the rest.

Also this maintains the behaviour as it was prior to the mentioned PR.

[nvahalik]

Yikes. This has broken at least one 3rd party package that depending on shift() returning null rather than an empty collection! laravel-json-api/laravel#287

[Treggats]

Yikes. This has broken at least one 3rd party package that depending on shift() returning null rather than an empty collection! laravel-json-api/laravel#287

Same, our tests started failing. So hence this PR. Just waiting on the release. In the mean time we updated our codebase to use the native array_shift(..)

[driesvints]

I just cut a new release that includes this one.