[13.x] Solve another PHP 8.5 null index deprecation

[tontonsb]

There have been a few fixes already. On two cases I'm still encountering warnings:

• $collection->keyBy('someKey') emits a warning if some values in someKey are null.
• $collection[$offset] emits a warning if $offset = null.

I propose to solve the Collection::keyBy issue by doing a cast to string (effectively replacing it with an empty string) because that's

• how it used to work and still works
• what Collection::groupBy does since solving the same issue in [12.x] Fix PHP 8.5 null-key deprecations #57137

I think it should be solved in framework for convenience, backwards compatibility and parity with the Collection::groupBy behaviour. I am also adding a simple test for both keyBy and groupBy to make this behaviour more locked in.

What about $collection[$offset]?

That warning is easier to solve on your own and it forces you to focus the attention on auto-cast in exactly the way that the warning was intended to do. So I haven't tried to solve it yet. But I've done some research:

• collect(['a' => 1, '' => 3])->has(null) is true
• collect(['a' => 1, '' => 3])->get(null) returns 3
• collect(['a' => 1, '' => 3])[null­] returns 3 and emits the warning

So maybe we should make ->get(null) and [null] work the same while we're at it? Is dropping the warning the correct direction here?

By the way

• Arr::exists(['a' => 1, '' => 3], null) is true
• Arr::has(['a' => 1, '' => 3], null) is false 🙃
• Arr::get(['a' => 1, '' => 3], null) returns ['a' => 1, '' => 3] (the whole array)