luci-base: uci.js: prevent sending empty uci set operations

Under certain circumstances, a staged uci option value might get unset
again before saving the config, leaving an empty section change set
behind, causing the save call to send an empty uci set request via rpc,
triggering an ubus code 4 (Resource not found) error.

In particular this prevented bridge VLANs from getting saved properly.

Fixes: #5876
Signed-off-by: Jo-Philipp Wich <jo@mein.io>