counsel.el (counsel-yank-pop): ignore entirely empty candidates

[braham-snyder]

• whose most noticeable effect -- when present in kill-ring and
  not ignored here -- is their preselection upon every new call to
  counsel-yank-pop (since they exactly match an empty initial
  input); empty initial input could perhaps be special-cased w/r/t
  preselection, but, regardless:

• pasting an entirely empty string is a no-op, so counsel-yank-pop
  should likely exclude such candidates (are entirely empty strings
  ever useful ivy candidates?)

[abo-abo]

Thanks.