Don't assert in wxOSX if a shaped window doesn't define a valid shape.

A window with wxFRAME_SHAPED style can still not defined any valid shape
actually, in which case wxFRAME_SHAPED should be just ignored.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76681 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Show the shaped frame with start shape initially in the sample.

It's not clear that setting the window shape actually works if the initial
shape is not set.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76682 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775