Let $enableBranchNode be a Closure or null

[gp-lnuff]

Hello,

I've found myself using this package and needing the branch nodes to dynamically be selectable or not (namely, to be selectable when the selectTree is multiple choice, and not to be when it's not).
I thought it was quite odd that $enableBranchNode was type hinted to be strictly a bool, when the method getEnableBranchNode() already evaluates its value.

I changed it locally and it was working, so I though to make a pull request.
I would like to add tests, but they don't seem to be working out of the box, so if you could guide me on how to add them that would be most appreciated.

[CodeWithDennis]

Allowing a closure makes sense; however, I don't think making it nullable does...

[gp-lnuff]

I thought making it nullable would put it in line with most others "closure-able" properties in Filament components but after looking through Filament's source again I see that Closure|bool properties with a default bool value tend not to be nullable. I'll change that

[CodeWithDennis]

Thanks!