Don't rewrite (nconc X nil) -> X for any X (bug#63103)

Don't rewrite (nconc X nil) -> X for any X (bug#63103)

Since the last cdr of a non-terminal argument to `nconc` is
overwritten no matter its value:

  (nconc (cons 1 2) nil) => (1)

a terminating nil arg cannot just be eliminated unconditionally.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-nconc):
Only eliminate a terminal nil arg to `nconc` if preceded by
a nonempty proper list.  Right now we only bother to prove this
for `(list ...)`, so that

  (nconc (list 1 2 3) nil) -> (list 1 2 3)

* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--test-cases): Add test cases.