ivy-use-selectable-prompt doesn't work when a dynamic collection is empty

[Stebalien]

When a dynamic collection is empty, one can't select the input field by enabling ivy-use-selectable-prompt.

Also, pressing RET calls the action on the last highlighted item (the item that was being highlighted just before the dynamic collection function returned an empty collection). This is probably related so I figured I'd note it.

[fabacino]

Could reproduce the issue with

(ivy-read "choose: "
          '(lambda (x)
             (if (> (length x) 2)
                 '()
               '("a" "b" "c")))
          :dynamic-collection t)

I will take a look at it.

[abo-abo]

@fabacino Sorry, I have to revert your commit: it breaks the smooth redisplay for :dynamic-collection. The minibuffer is blinking rapidly as the user types. We'll have to figure out how to fix this.

[fabacino]

@abo-abo Thanks for your feedback. I think the problem is that a value of nil for ivy--all-candidates can mean two different things:

1. The function (sync or async) has returned no candidates.
2. We don't know the candidates yet, because an async function is still running.

I made a new fix, basically checking if there is a running counsel process. If there is, an async function is still running and we don't refresh the minibuffer. If there is none, however, we can safely assume that the function has returned an empty set of candidates. In that case, we update the minibuffer.

fabacino@b7f2700

If you like it, I can PR. The hard-coded process name is not pretty, but there are other references to the counsel package within ivy.el, so I figured it is probably ok.

[abo-abo]

If you like it, I can PR. The hard-coded process name is not pretty, but there are other references to the counsel package within ivy.el, so I figured it is probably ok.

Sounds good, please PR. We can refactor the hard-coded stuff later if there's a need.