_get_tg_setting() broken with set -u

Originally created by @anonym on #10785 (Redmine)

I saw this in the journal:

Dec 21 15:53:40 amnesia add-GNOME-bookmarks[3543]: /usr/local/lib/add-GNOME-bookmarks: 1: eval: TAILS_PERSISTENCE_READONLY: parameter not set

Indeed, tails-greeter.sh’s _get_tg_setting() (via persistence_is_enabled_read_write()) is not adapted for set -u.

Feature Branch: bugfix/10785-_get_tg_setting-vs-set-u