This was more difficult to solve than I expected. The main issue is that the
`[open]` attribute on `#urlbar` wasn't accurate when the view was "open" but
there were no results or one-offs, so it was in fact closed. This broke a few
style rules.

This bug can also be reached when the user has no engines and clears the search
string while in search mode. This includes pressing Accel+K when typing a search
string while not in search mode.

The relationship between the UrlbarView and the one-offs is complex and depends
on a lot of listeners and async calls made in synchronous contexts. Furthermore,
most of the UrlbarView open/close code is synchronous, but checking the number
of engines (to determine if the one-offs will open) is an async operation. These
factors make it difficult for the view to discern any state about the one-offs
and plan accordingly.

First I tried adding a [oneoff] attribute to .urlbarView, set asynchronously when
the one-offs are built. Then I changed CSS rules to check
```
.urlbarView:not([noresults]),
.urlbarView[oneoffs]
```
instead of just `#urlbar[open]`. This approach would've required changing
a bunch of CSS from the simple `#urlbar[open]` to the more complicated CSS
above, which was not good for code readability. Also it would keep `[open]` in
a weird useless state where it couldn't really be trusted. This would've caused
other styling problems.

I settled on adding a `.then` call around the affected UrlbarView opening. The
view opens only if we are sure that we will have either results or one-offs,
so we can once again trust the `[open]` parameter. This approach has its
drawbacks. Mainly, it's a more JavaScript-heavy solution so it's possibly
slow. Thankfully, it's something that can be easily cached.

Differential Revision: https://phabricator.services.mozilla.com/D92526