Fix unused client removal on restarted container

Handle the case where the target does not exist
in the process of deleting unused clients.
Such a situation will not occur in newly launched containers,
but will occur if they are restarted.

During container restarts, container status are preserved.
If the unused database client was deleted in the last run,
grep will not match anything.
It returns non-zero code and the container stops there
because entrypoint sets option `-e`
(exit immediately on non-zero exit code excluding some special cases)

This commit make the uninstall process to handle the case
UNUSED_DB_CLIENTS is empty.

assets/runtime/functions

@@ -237,7 +237,16 @@ gitlab_uninstall_unused_database_client() {
     REGEX_DB_CLIENT_VERSIONS_IN_USE="-common${DB_CLIENT_VERSIONS_IN_USE}"
 
     # remove unused client using regex above
-    UNUSED_DB_CLIENTS=$(apt-cache pkgnames postgresql-client | grep -v -e "${REGEX_DB_CLIENT_VERSIONS_IN_USE}" | tr '\n' ' ')
+    # grep may return non-zero code on mo match, so fake the exit code with the `|| true` to swallow that
+    UNUSED_DB_CLIENTS=$(apt-cache pkgnames postgresql-client | grep -v -e "${REGEX_DB_CLIENT_VERSIONS_IN_USE}" || true)
+    if [[ "${UNUSED_DB_CLIENTS}" == "" ]]; then
+      echo "- All installed version of clients are in use. Did not uninstalled any client..."
+      return
+    fi
+
+    # just to get clean log, convert newline (package name delimiter) to single whitespace
+    UNUSED_DB_CLIENTS=$(echo ${UNUSED_DB_CLIENTS} | tr '\n' ' ')
+
     echo "- Uninstalling unused client(s): ${UNUSED_DB_CLIENTS}"
     DEBIAN_FRONTEND=noninteractive apt-get -qq -y purge -- ${UNUSED_DB_CLIENTS} >/dev/null
   fi