I hit this bug when I called `M-x
denote-change-file-type-and-front-matter` on an Org file with empty
filetags, to convert it to Markdown-YAML format.

This is the front-matter of the original file:
```
   #+title:      Great Content is the Heart of Audience Building
   #+filetags:
   #+date:       [2023-03-08 Wed 18:51]
   #+identifier: 20230308T185100
```

The stack-trace is as follows:
```
Debugger entered--Lisp error: (error "‘’ is not a list")
  signal(error ("‘’ is not a list"))
  error("`%s' is not a list" "")
  denote-sluggify-keywords("")
  denote--format-front-matter-keywords("" markdown-yaml)
  denote--format-front-matter("Great Content is the Heart of Audience Building" "2023-03-08T18:51:00+05:30" "" "20230308T185100" markdown-yaml)
  denote--add-front-matter("/.../20230308T18..." "Great Content is the Heart of Audience Building" "" "20230308T185100" markdown-yaml)
  denote-change-file-type-and-front-matter("/.../20230308T18..." markdown-yaml)
```

Following the chain of commands shows that the culprit is the line
```
(denote-retrieve-keywords-value file old-file-type)
```
in `denote-change-file-type-and-front-matter`, which returns an
empty string when the rest of the functions expect a list of strings.

I looked at where `denote-extract-keywords-from-front-matter` is being
used and why the code was originally returning an empty string, and it
looks to me that returning an empty list from it now is the right
thing to do.
---
 denote.el | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/denote.el b/denote.el
index 5796c69..88fdcbf 100644
--- a/denote.el
+++ b/denote.el
@@ -1129,11 +1129,11 @@ for how this is used."
 (defun denote-extract-keywords-from-front-matter (keywords-string)
   "Extract keywords list from front matter KEYWORDS-STRING.
 Split KEYWORDS-STRING into a list of strings.  If KEYWORDS-STRING
-satisfies `string-blank-p', return an empty string.
+satisfies `string-blank-p', return an empty list.
 
 Consult the `denote-file-types' for how this is used."
   (if (string-blank-p keywords-string)
-      ""
+      '()
     (split-string keywords-string "[:,\s]+" t "[][ \"']+")))
 
 (defvar denote-file-types
-- 
2.42.0

> From: Vedang Manerikar <ved.manerikar@gmail.com>
> Date: Sun,  5 Nov 2023 16:13:13 +0530

> [... 37 lines elided]

>  denote.el | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Thank you, Vedang! I just installed your patch.

-- 
Protesilaos Stavrou
https://protesilaos.com