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