Don't show matches with no input for nil icomplete-show-matches-on-no-input

Don't show matches with no input for nil icomplete-show-matches-on-no-input

* lisp/icomplete.el (icomplete-show-matches-on-no-input): Fix docstring.
(icomplete--initial-input): New internal variable.
(icomplete-minibuffer-setup): Set buffer-local
icomplete--initial-input to icomplete--field-string.
(icomplete-ret, icomplete-force-complete-and-exit)
(icomplete--sorted-completions, icomplete-exhibit):
Compare icomplete--initial-input with icomplete--field-string
to detect no input.  (Bug#19031)

etc/NEWS: Remove duplicate entry.