Restore service triggers which got wrongly removed in commit
733aae9584 ("fix issues").

Without triggers, changing settings from LuCI or calling reload_config
from the cli won't have any effect.

Also adjust the uci commit call to only commit the minidlna configuration
when setting a UUID, to avoid committing unrelated user changes in other
config files.

Ref: https://github.com/openwrt/luci/issues/4194
Fixes: 733aae958 ("minidlna: fix issues")
Fixes: 37367bdc8 ("minidlna: create UUID in config if it is empty")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>