Let wxWindow::Fit[Inside]() work even for windows without children.

Let wxWindow::Fit[Inside]() work even for windows without children.

The best [virtual] size of the window is usually determined by its children
but it's also possible to have a min size constraint set on the window sizer
so set the window to its best size in Fit() independently of whether it has
children or not.

Closes #14668.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72507 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775