Merge pull request 'Fix verification string computation.' (legoscia#38)…

… from hdasch/emacs-jabber:fix-caps-ver-string into production

Fix issue legoscia#37, a runtime error generally masked unless running non-nil debug-on-error.  Even then, this is an infrequent error provoked by Psi+ clients.

Intention to merge mentioned on xmpp:jabber-el.

Reviewed-on: https://codeberg.org/emacs-jabber/emacs-jabber/pulls/38

lisp/jabber-disco.el

@@ -305,7 +305,7 @@ obtained from `xml-parse-region'."
 		;; For each <value/> element, append the XML character
 		;; data, followed by the '<' character.
 		(dolist (value values)
-		  (insert value "<"))))))))
+		  (insert (or value "") "<"))))))))
 
     ;; 8. Ensure that S is encoded according to the UTF-8 encoding
     ;; (RFC 3269 [18]).