Allow zero-argument rx `or' and `seq' forms

Make the rx `or' and `seq' forms accept zero arguments to produce a
never-matching regexp and an empty string, respectively.

* lisp/emacs-lisp/rx.el: Require cl-extra.
(rx-constituents, rx-or): Permit zero args.
(rx): Amend doc string for `or' and `seq'.
* test/lisp/emacs-lisp/rx-tests.el (rx-or, rx-seq): Test the change.
* etc/NEWS (Changes in Specialized Modes and Packages): Mention the change.
1 job for master in 52 minutes and 5 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #1866


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:957: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1