[12.x] Add default null parameter to reorderDesc()

[amirhshokri]

This makes reorderDesc() consistent with reorder() by allowing it to be called without arguments.

[rodrigopedra]

Does it make any sense to call Builder@reorderDesc() without any parameter?

Wouldn't it be easier to call Builder@reorder()?

When Builder@reorder() without a $column parameter, it clears any order by clauses from the builder and doesn't add any new order by clauses.

Calling Builder@reorderDesc(null), or with a default null value as this PR proposes, is not different from just calling Builder@reorder().

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!