counsel-company invokes ivy-read only if there are candidates

[kisp]

Hi, this is a small change to counsel-company that I think is generally useful.

In my use case, I am using counsel-company as a completion-at-point-function. With my proposed change, I am enjoying to see the message "No completion found" instead of being prompted by ivy with no candidates available.

I am setting it up like this:

(defun my--completion-at-point ()
  #'counsel-company)

(setq-local completion-at-point-functions '(my--completion-at-point t))