fix(ui): don't try to print null

[stolendata]

g_strdup_printf() bottoms out to vfprintf(), which on OpenBSD throws a syslog notice when fed a null pointer, resulting in Profanity generating endless log spam on OpenBSD.

Sep 17 16:37:52 ohdruid profanity: vfprintf %s NULL in "%s/"
Sep 17 16:37:52 ohdruid last message repeated 2 times
Sep 17 19:09:13 ohdruid profanity: vfprintf %s NULL in "%s/"
Sep 17 19:17:51 ohdruid last message repeated 4 times
Sep 17 20:22:52 ohdruid profanity: vfprintf %s NULL in "%s/"