luci-base: fix uci write bug when input value equals default

luci-base: fix uci write bug when input value equals default

Avoid executing unnecessary write operations to flash/UCI when the
user's form selection or input exactly matches the component's
`default` configuration.

Previously, if a field had a valid value identical to its default,
it would bypass the empty check and trigger a `write()` call, leading
to configuration bloating. This change ensures that such values are
correctly intercepted and handled via `this.remove()` to keep the
underlying configuration file clean and precise.

Signed-off-by: Andy Chiang <redacted>