Can't unset the last blocktheme

To reproduce:

1. Set the blocktheme for one block
2. Notice that everything works fine
3. Unset the blocktheme for that block by selecting <-- None -->
4. Notice that the blocktheme doesn't get removed

The problem is in blocktheme_set() where $new_val is checked before setting the blocktheme variable. When the last blocktheme is removed, $new_val is empty, so this fails to remove the last setting.