Use denote-link-markdown-follow only if there is a link

Otherwise, we are breaking general markdown-mode functionality.

Thanks to pmenair for bringing this matter to my attention in issue
290: <https://github.com/protesilaos/denote/issues/290>.

README.org

@@ -5339,7 +5339,7 @@ Denote is meant to be a collective effort.  Every bit of help matters.
   Monnier, Stefan Thesing, Thibaut Benjamin, Tomasz Hołubowicz, Vedang
   Manerikar, Wesley Harvey, Zhenxu Xu, arsaber101, ezchi, jarofromel,
   leinfink (Henrik), l-o-l-h (Lincoln), mattyonweb, maxbrieiev,
-  mentalisttraceur, relict007.
+  mentalisttraceur, pmenair, relict007.
 
 + Ideas and/or user feedback :: Abin Simon, Aditya Yadav, Alan
   Schmitt, Aleksandr Vityazev, Alex Hirschfeld, Alfredo Borrás, Ashton

denote.el

@@ -3954,9 +3954,9 @@ positions, limit the process to the region in-between."
 (defun denote-link-markdown-follow (link)
   "Function to open Denote file present in LINK.
 To be assigned to `markdown-follow-link-functions'."
-  (string-match denote-id-regexp link)
-  (funcall denote-link-button-action
-           (denote-get-path-by-id (match-string 0 link))))
+  (when (ignore-errors (string-match denote-id-regexp link))
+    (funcall denote-link-button-action
+             (denote-get-path-by-id (match-string 0 link)))))
 
 (eval-after-load 'markdown-mode
   '(add-hook 'markdown-follow-link-functions #'denote-link-markdown-follow))