Fix types in various defcustom declarations

* lisp/org/ob-clojure.el
(ob-clojure-babashka-command, ob-clojure-nbb-command):
Account for the possibility of nil (commands absent).
* lisp/org/org.el (org-safe-remote-resources):
* lisp/org/ox-md.el (org-md-toplevel-hlevel):
* lisp/progmodes/project.el (project-vc-extra-root-markers):
Fix incorrect or invalid types.

lisp/org/ob-clojure.el

@@ -101,13 +101,13 @@
 
 (defcustom ob-clojure-babashka-command (executable-find "bb")
   "Path to the babashka executable."
-  :type 'file
+  :type '(choice file (const nil))
   :group 'org-babel
   :package-version '(Org . "9.6"))
 
 (defcustom ob-clojure-nbb-command (executable-find "nbb")
   "Path to the nbb executable."
-  :type 'file
+  :type '(choice file (const nil))
   :group 'org-babel
   :package-version '(Org . "9.6"))
 

lisp/org/org.el

@@ -1408,7 +1408,7 @@ URI regexps are applied to both URLs and Org files requesting
 remote resources."
   :group 'org
   :package-version '(Org . "9.6")
-  :type '(list regexp))
+  :type '(repeat regexp))
 
 (defcustom org-open-non-existing-files nil
   "Non-nil means `org-open-file' opens non-existing files.

lisp/org/ox-md.el

@@ -87,7 +87,7 @@ included into another document or application that reserves top-level
 headings for its own use."
   :group 'org-export-md
   :package-version '(Org . "9.6")
-  :type 'string)
+  :type 'natnum)
 
 
 

lisp/progmodes/project.el

@@ -453,7 +453,7 @@ repositories.
 
 In either case, their behavior will still obey the relevant
 variables, such as `project-vc-ignores' or `project-vc-name'."
-  :type 'list
+  :type '(repeat string)
   :version "29.1"
   :package-version '(project . "0.9.0")
   :safe (lambda (val) (and (listp val) (cl-every #'stringp val))))