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

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 <[email protected]>
(cherry picked from commit 25dcd0b77c4450b83cd8b8307397e58c34ebfcd1)