Keep the button bitmap centered if the button has no label in wxMSW.

Keep the button bitmap centered if the button has no label in wxMSW.

We should only honour the bitmap alignment if the button shows both the bitmap
and the label. If only the bitmap is shown (e.g. when wxBitmapButton is used),
it should always be centered as it used to be done in 2.8.

Closes #12323.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65235 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775