gluon-core: adjust to new escaped array handling for generate expressions

package/gluon-core/generate/lib/gluon/upgrade/core/invariant/011-ntp-servers

 #!/bin/sh
 
-[ -n '@ntp_servers@' ] || exit 0
+[ -n "$(echo @ntp_servers@)" ] || exit 0
 
 
 uci delete system.ntp.server