appinfo: avoid overriding the system defaults when adding support
We want to be compatible with the following situation:
- there's no explicit default set in mimeapps.list
- we add support for a content type to a specific application, and that
list is empty
- the default should be picked from the system list, not overridden by
the user-added application.
So we make the default explicit in this case, by adding it to the
relevant section in mimeapps.list.
https://bugzilla.gnome.org/show_bug.cgi?id=637675