ITS#9353 fix monitor_back_register_database for empty suffix DB

Use the correct database entry instead of the frontendDB entry
3 jobs for its9353 in 31 minutes and 36 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #2104
build-gnutls-mit

00:28:02

passed #2102
build-no-threads-no-slapd

00:03:33

passed #2103
build-openssl-heimdal

00:29:22