Allow retrieving the descent and external leading of empty strings.

This used to work before wxTextMeasure changes so make it work again instead
of optimizing the case of empty string away and not returning anything in the
descent and externalLeading output parameters in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74464 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775