bug#50965: Allow nil third argument for mapconcat

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Sun, 3 Oct 2021 01:03:54 +0200
> Cc: 50965@debbugs.gnu.org
> 
> +Optional argument SEPARATOR must be a string, a vector, or a list of
> +characters; nil is the empty string."

nil is not a string, so saying that is not the best wording.  I
suggest "nil stands for an empty string." instead.

> +Optional argument SEPARATOR must be a string, a vector, or a list of
> +characters; nil is the empty string.

Likewise.

> +(ert-deftest fns-tests-mapconcat ()
> +  (should (string= (mapconcat #'identity '() "_") ""))
> +  (should (string= (mapconcat #'identity '("A") "_") "A"))
> +  (should (string= (mapconcat #'identity '("A" "B") "_") "A_B"))
> +  (should (string= (mapconcat #'identity '("A" "B" "C") "_") "A_B_C"))
> +  (should (string= (mapconcat #'identity '("a" "b")) "ab")))

Please be sure to test with vectors, bool-vectors, list of characters,
and non-ASCII strings as well.

> I'm assuming this should go to master at this point.

Yes, thanks.