Make denote--prepare-note only error if an existing file is not empty

This way we avoid cases where some other function saves the buffer
before we get to operate on it. One such example is 'lsp-auto-touch-files',
as noted by wuzhihao in issue 627: <https://github.com/protesilaos/denote/issues/627>.

README.org

@@ -6458,7 +6458,7 @@ Denote is meant to be a collective effort.  Every bit of help matters.
   fingerknight, hpgisler, hyperfocus1337,johkneisl, jtpavlock, juh,
   leafarbelm, mentalisttraceur, mjkalyan, oatmealm, pRot0ta1p,
   rbenit68, relict007, sarcom-sar, sienic, skissue, sundar bp,
-  yetanotherfossman, zadca123
+  wuzhihao, yetanotherfossman, zadca123
 
 Special thanks to Peter Povinec who helped refine the file-naming
 scheme, which is the cornerstone of this project.

denote.el

@@ -2917,8 +2917,11 @@ Arguments TITLE, KEYWORDS, DATE, ID, DIRECTORY, FILE-TYPE,
 TEMPLATE, and SIGNATURE should be valid for note creation."
   (let* ((path (denote-format-file-name
                 directory id keywords title (denote--file-extension file-type) signature))
-         (buffer (if (file-regular-p path)
-                     (user-error "A file named `%s' already exists" path)
+         ;; NOTE 2025-08-02: Is it safe to assume that an
+         ;; existing+empty file is good for us to use?  Otherwise, we
+         ;; should have a `y-or-n-p' prompt here.
+         (buffer (if (and (file-regular-p path) (not (denote--file-empty-p path)))
+                     (user-error "A file named `%s' already exists and is not empty" path)
                    (find-file path)))
          (header (denote--format-front-matter title date keywords id signature file-type)))
     (with-current-buffer buffer