org-priority: Fix SPC handling for numeric priorities (bug#52341)

* lisp/org.el (org-priority): When reading numeric priorities, provide
special handling of SPC so that it clears the priority, as advertised.

If the user has configured numeric priorities and tries to clear a
priority by inputting SPC at the prompt, org-priority feeds " " to
string-to-number and ends up with 0 instead of the ?\s (32) that's
used downstream to signal "remove".  Explicitly check for " " and
translate it to ?\s.

Reported-by: "Bruce E. Robertson" <brucer42@gmail.com>
Link: https://list.orgmode.org/877dch89s1.fsf@kyleam.com/

lisp/org.el

@@ -11323,13 +11323,14 @@ or a character."
 	    (setq
 	     new
 	     (if nump
-                 (let ((msg (format "Priority %s-%s, SPC to remove: "
-				    (number-to-string org-priority-highest)
-				    (number-to-string org-priority-lowest))))
-                   (if (< 9 org-priority-lowest)
-		       (string-to-number (read-string msg))
-                     (message msg)
-                     (string-to-number (char-to-string (read-char-exclusive)))))
+                 (let* ((msg (format "Priority %s-%s, SPC to remove: "
+                                     (number-to-string org-priority-highest)
+                                     (number-to-string org-priority-lowest)))
+                        (s (if (< 9 org-priority-lowest)
+                               (read-string msg)
+                             (message msg)
+                             (char-to-string (read-char-exclusive)))))
+                   (if (equal s " ") ?\s (string-to-number s)))
 	       (progn (message "Priority %c-%c, SPC to remove: "
 			       org-priority-highest org-priority-lowest)
 		      (save-match-data