Fix assert when creating wxBitmapButton without a valid bitmap in wxMSW.

Fix assert when creating wxBitmapButton without a valid bitmap in wxMSW.

wxBitmapButton can be created without a valid bitmap if SetBitmapLabel() is
called later, so don't call SetBitmapLabel() from the constructor if no bitmap
was provided.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775