stackable-box: Add a missing null check

I must admit that it's unlikely to happen, and perhaps that's why nobody have noticed it until now.

The function hdy_stackable_box_size_allocate_unfolded () lacks a check if :visible-child is null or not, which causes that when instantiating a child-less HdyLeaflet, the app crashes with a segfault.

My commit just adds that missing check.