Don't show empty sizers.

Don't show empty sizers.

Revert the change of r44514 and do hide the empty sizers. Code relying on them
being shown/positioned should use wxRESERVE_SPACE_EVEN_IF_HIDDEN flag but
assuming it by default results in completely unexpected layouts, e.g. still
using margins around a sizer with hidden window if it contains an empty
sub-sizer.

Closes #11426.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65079 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775