Deb: Create empty migrated-from-my.cnf-settings.conf for Buildbot tests

The buildbot.askmonty.org has explicit tests that check that this file
exists, thus to get tests pass, an empty placeholder must be created.
Remove this once CI has been updated not to expect this file.

debian/mariadb-server-10.5.postinst

@@ -198,6 +198,12 @@ EOF
       fi
     fi
 
+    # @TODO: Remove once buildbot.askmonty.org has been updated not to expect this file
+    mkdir -p /etc/systemd/system/mariadb.service.d/
+    # Note that file cannot be empty, otherwise systemd version in Ubuntu Bionic
+    # will think the service is masked
+    echo "# empty placeholder" > /etc/systemd/system/mariadb.service.d/migrated-from-my.cnf-settings.conf
+
   ;;
 
   abort-upgrade|abort-remove|abort-configure)