If LC_ALL or LC_MESSAGES is set but empty, ignore it and use the next
environment variable. This is consistent with the standard setlocale()
behaviour which only uses LC_ALL if it is set and not empty.
Closes#15006.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73466 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775