Exclude empty fields from blocks JSON#358

Whenever you add a new field to a block’s blueprint and save a page that uses this block, the new field is added with an empty value.

This unnecessarily bloats the TXT file and hinders Git tracking because new “changes” appear even though the panel user hasn’t changed anything. This doesn’t sound like much, but when you have a couple of weeks worth of changes and you want to merge them with something else you were working on, you get bombarded with conflicts and you don’t know what’s what. Not a pleasant experience.