luci-mod-network: handle missing dnsrr hexdata as empty string

luci-mod-network: handle missing dnsrr hexdata as empty string

If the value returned from UCI was falsy, it became an empty array instead
of an empty string, which crashed the UI when trying to invoke the
non-existing replace instance function on the value.

Signed-off-by: Daniel Nilsson <dannil+github@protonmail.com>