This is a translation of commit 363134d28cb7ff8d2313278e1b4b6c09b5bf9bed
from CoffeeScript to JavaScript. Original commit message from
363134d28cb7ff8d2313278e1b4b6c09b5bf9bed:

When saving a draft to the drafts store, don't save changes for fields
which aren't actually set. This prevents us from accidentally later
restoring `null` to one of these fields.

Fixes an issue where creating an annotation when logged-out would result
in a null permissions field.