Let append and prepend be set to null without throwing an exception

[gp-lnuff]

Hello,

the $append/$prepend properties are already nullable, however when you set them to null through the setter methods it triggers the InvalidArgumentException.
I would not consider this expected behaviour, especially when passing a Closure (which is allowed) that conditionally appends or prepends an item based on a condition, like user permissions.
By adding a single clause that does nothing if null is passed in (or if $prepend/$append evaluate to null), expected behaviour is restored.
Let me know if there is a reason it should behave like this, thank you.