[10.x] When should return the callback value rather than the conditionable object

[eduance]

This PR addresses the issue in #44712

When using the when() on the Collection instance, it used the null-coalescing operator to return the conditionable concern rather than simply returning the callback.

This caused multiple issues, for example;

        $collection = collect([
            ['name' => 'John'],
            ['name' => 'Paul'],
        ]);

        $value = $collection->when(true, fn ($collection) => $collection->firstWhere('name', 'Taylor'); ); 

This causes the value to be null, leaving the null-coalescing operator to return the $this; which was not really the expected behavior. You would want the value to just return whatever the method returns you instead.

This could be a really nice upgrade for 10.x so the when method doesn't all of a sudden take over the responsibility of returning the correct value.