Correct regexp-opt return value for empty string list

Correct regexp-opt return value for empty string list

When regexp-opt is called with an empty list of strings, return a regexp
that doesn't match anything instead of the empty string (Bug#20307).

* doc/lispref/searching.texi (Regular Expression Functions):
* etc/NEWS:
Document the new behaviour.
* lisp/emacs-lisp/regexp-opt.el (regexp-opt):
Return a never-match regexp for empty inputs.