Fixed return type for RedistList::get()

[Bilge]

This fixes a bug that caused the following exception to be thrown when the list key did not exist:

TypeError : Amp\Redis\Command\RedisList::get(): Return value must be of type string, null returned

As a bonus, we expanded the allowed types for RedistList::get to include integer, which is probably the type it should have always been. We don't replace string at this time, though, for BC reasons.

[trowski]

Thanks!