Fix icomplete-force-complete-and-exit for no-input situations

Fix icomplete-force-complete-and-exit for no-input situations

If there is no minibuffer input, but the user has already cycled some
pre-calculated completions, we should be calling
minibuffer-force-complete-and-exit instead of
minibuffer-complete-and-exit.  The former is guaranteed to be fast in
this situation and yields the desired "selected" completion, while the
latter will just give us the default, ignoring all the cycling of
icomplete-{forward|backward}-completions.

* lisp/icomplete.el (icomplete-force-complete-and-exit): Add
comments and fix for empty input but some completions calculated.