read-char-from-minibuffer for optional argument

Question

The following counts the number of opening (brcmk-beg) and closing (brcmk-end) braces. Having brcmk-end an optional argument, I would like to print

$begmk nil $count-begmk 0

when the user either hits Return or uses a blank space when using the minibeffer with the call (read-char-from-minibuffer "Closing Bracemark: ").

    (cl-defun rk-bracemark-count-from-char
        (brcmk-beg &optional brcmk-end
                             (rgn-beg (region-beginning))
                             (rgn-end (region-end)))
      "Count number of opening and closing brace marks.
    Search is applied within a region."
    
      (interactive
       (list (read-char-from-minibuffer "Opening Bracemark: ")
             (read-char-from-minibuffer "Closing Bracemark: ")
             (region-beginning)
             (region-end) ))
    
      (save-excursion
        (let ( ($count-begmk  0) ($count-endmk  0) 
               ($begmk  (char-to-string brcmk-beg))
               ($endmk  (or (and brcmk-end (char-to-string brcmk-end))
                "nil")) )
    
          (goto-char rgn-beg)
          (while (and (< (point) rgn-end)
                      (search-forward $begmk rgn-end t))
            (cl-incf $count-begmk))
    
          (goto-char rgn-beg)
          (while (and brcmk-end
                  (< (point) rgn-end)
                  (search-forward $endmk rgn-end t))
            (cl-incf $count-endmk))
    
          (message "Marks: %s %s  Counts: %d %d" $begmk $endmk
               $count-begmk $count-endmk)
          (goto-char rgn-beg) )))

Answer 1

(require 's) ; for s-blank-str-p

...
  (interactive
    (list (read-char-from-minibuffer "Opening Bracemark: ")
          (let ((x (read-char-from-minibuffer "Closing Bracemark: ")))
            (unless (s-blank-str-p (char-to-string x)) x))
          (region-beginning)
          (region-end)))
  ;; at this point brcmk-end is either nil or a closing delimiter
...