luci-mod-status: channel_analysis.js: fix stale scan result.

When the new result is empty, including when another scan is running in the background, the table was emptied, but the graph persisted.
With this change, the list will persist and missing APs will be shown faded.

Signed-off-by: Milad Mohtashamirad <miladmohtashamirad@gmail.com>