When an app field is empty, don't hide all the translations (bug 888831)

https://bugzilla.mozilla.org/show_bug.cgi?id=888831

When we had an empty translation for a field, whatever the reason, we replaced the whole field, including translations in other languages, by a span. It turns out it's a bad idea since it will hide other translations, even non-empty ones, causing a lot of confusion since the edit form itself displays correctly.

WIP, I expect this to break other tests but I wanted some initial feedback about the idea.