Don't create a valid wxRegion when using default ctor in wxOSX.

Don't create a valid wxRegion when using default ctor in wxOSX.

Default constructing a wxRegion created an object that unexpectedly passed
IsOk() test in wxOSX. This was completely unexpected so don't do this and
leave default constructed wxRegion invalid, as in the other ports.

Update DoCombine() to be able to deal with the case of an invalid source
region in a way consistent with the other ports.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69459 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775