fix layer rename leaving dangling specifications

The asset manager finishes processing changes by iterating the specifications object .  There is working code to remove components if a layer name was ‘player.png’ and is now just ‘player’...

https://github.com/adobe-photoshop/generator-assets/blob/jhatwich/layer-update/lib/assetmanager.js#L416

...but leaving empty arrays out of the specifications object prevented it from running.