Fix plugin search with GLib 2.69.

Named groups in `GRegex` with GLib 2.69 appear to return empty, not `NULL` strings, so handle that when running version comparison.

This is actually what GLib documents should happen, but I guess didn't work right before.

Testing Done:
Compiled and ran tests on Fedora Rawhide with GLib 2.69.

Reviewed at https://reviews.imfreedom.org/r/856/