specify no toolbox with <xml></xml>

The way that blockly parses properties is to check to see if dashboard provided a truthy value, and if not, look to see if it has its own value. For "custom" levels, we provide a set of defaults (including a toolbox) that are used if the dashboard value is not truthy. I don't know if there are levels that depend on getting this default toolbox, but I suspect there could be.

This means if in levelbuilder, we give no toolbox for artist, we end up with the full toolbox. If we instead provide "<xml></xml>", we get not blocks in our toolbox. However, blockly still behaves differently with an empty toolbox vs. no toolbox.

Fix, which is a little bit hacky, is to special case <xml></xml> (or the simplified <xml/>), and in that case set our toolbox to null.