Allow unsetting of location tax on exhibit modal

[spelkey-ucd]

Currently, if a user wants to remove all location tax terms from an exhibit, it has to be done in the "quick edit" setting of the exhibits list admin page - since the "none" option is disabled.

To enable this functionality from the modal, im thinking we have to:

1. removed the disabled attribute from the select option
2. in this line https://github.com/UCDavisLibrary/ucdlib-wp-plugins/blob/main/ucdlib-special/src/editor/lib/plugins/exhibit.js#L344, return an empty array if id is empty