KF5I18NMacros: Don't install an empty dir when no po files exist

Summary:
I saw this happen with kdecoration from git since
64d9f92f6a8708814f414dda0bb0d0e91c27235f. The directory passed in
its ki18n_install call doesn't exist, resulting in an empty
directory (LOCALE_INSTALL_DIR) getting installed, which at least
some packaging systems don't like.

Test Plan:
Tested with kdecoration from git and tarballs where podir
exists

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12905