exports: Revert a minor unintended behaviour change
Before commit a495f1ee, we would have returned false here, but since a495f1ee we would fall through to replacing the symlink with its target.
Fixes: a495f1ee "exports: Add flatpak_debug2() logging at a finer granularity"
This syncs up with the version proposed upstream at https://github.com/flatpak/flatpak/pull/5010.