Handle invalid/empty enum values in store files better
#0 __strrchr_avx2 at ../sysdeps/x86_64/multiarch/strrchr-avx2.S:230
#1 g_log_writer_journald at gmessages.c:2529
#2 bolt_log_journal at ../boltd/bolt-log.c:520
#3 daemon_logger at ../boltd/bolt-daemon.c:82
#8 g_return_if_fail_warning at gmessages.c:2809
#9 g_enum_get_value_by_nick at genums.c:461
#10 bolt_enum_from_string at ../common/bolt-enums.c:99
#11 bolt_device_type_from_string at ../common/bolt-enums.c:306
#12 bolt_store_get_device at ../boltd/bolt-store.c:433
#13 bolt_manager_initialize at ../boltd/bolt-manager.c:613