Don't use DDEExec registry key in wxMSW wxExecute() if it's empty.

Don't use DDEExec registry key in wxMSW wxExecute() if it's empty.

Some file types have DDEExec subkey in the registry but no value for it, don't
use DDE for launching the files of these types in this case as this only
results in errors.

Closes #15388.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74636 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775