Fix M-j with default fill-prefix value

[Javier Olaechea <pirata@gmail.com>]

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org-comment-line-break-function does not handle fill-prefix being set to
nil, which is the default value for fill-prefix. This means that pressing
M-j inside an org-mode buffer in a vanilla installation of Emacs results in
an error. From looking at other callers of
insert-before-markers-and-inherit it is clear that a guard against
fill-prefix being nil is missing.

This has been reported beforehttps://
list.orgmode.org/87o861o9sh.fsf@aquinas.i-did-not-set--mail-host-address--so-tickle-me/

Cheers,
Javier Olaechea

-- 
"I object to doing things that computers can do." — Olin Shivers

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From 372daea6cbee60df03f1a68bd5fabca4212a12c3 Mon Sep 17 00:00:00 2001
From: Javier Olaechea <pirata@gmail.com>
Date: Wed, 20 Apr 2022 11:58:07 -0500
Subject: [PATCH] lisp/org.el: Allow org-c-l-b-f to handle an empty fill-prefix

* org.el (org-comment-line-break-function): When fill-prefix is nil,
don't call `insert-before-markers-and-inherit'.

The default value for fill-prefix is nil,
`insert-before-markers-and-inherit' expects a string. This results in
an error when calling `org-comment-line-break-function', bound to M-j,
on a vanilla installation of Emacs. We should add a guard checking
that fill-prefix is not nill before calling i-b-m-a-i. The only
exception is when adaptive-fill-mode is enabled. This is the approach
that that rest of the code in Emacs takes. Some call-sites listed
below:

* simple.el (default-indent-new-line): uses
  (and fill-prefix (not adaptive-fill-mode) ...)

* newcomment.el (comment-indent-new-line): uses
  (and fill-prefix (not adaptive-fill-mode) ...)

* fill.el (fill-new-line): uses
  (and fill-prefix (not (equal fill-prefix "")) ...)

TINYCHANGE
---
 lisp/org.el | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lisp/org.el b/lisp/org.el
index 2353c6594..4176c683f 100644
--- a/lisp/org.el
+++ b/lisp/org.el
@@ -19943,7 +19943,8 @@ (defun org-comment-line-break-function (&optional soft)
   (save-excursion (forward-char -1) (delete-horizontal-space))
   (delete-horizontal-space)
   (indent-to-left-margin)
-  (insert-before-markers-and-inherit fill-prefix))
+  (and fill-prefix (not adaptive-fill-mode)
+       (insert-before-markers-and-inherit fill-prefix)))
 
 
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