Avoid writing empty abbrev tables

Fixes bug#29923

'insert-abbrev-table-description' with a non-nil READABLE inserts Lisp
forms suitable for evaluation to restore the defined abbrevs.  We
don't have to insert a form for tables that do not have any abbrevs.

To implement this, we need to filter out system abbrevs before
checking if a table is empty, because system abbrevs were previously
skipped in the 'abbrev--write' call, at which point we would already
have started inserting the beginning of a table definition form.

* lisp/abbrev.el (insert-abbrev-table-description):
Skip inserting empty tables when READABLE is non-nil.
Clarify behavior in documentation string.
(abbrev--write): Remove system abbrev check.

* doc/lispref/abbrevs.texi (Abbrev Tables): Document behavior
with empty tables.

* etc/NEWS: Mention the change in behavior of
'insert-abbrev-table-description'.

1 files changed, 6 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 354072fc814..474af167e83 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -247,6 +247,12 @@ case does not match.
 for abbrevs that have them.
 
 +++
+** 'insert-abbrev-table-description' skips empty tables.
+'insert-abbrev-table-description' skips inserting empty tables when
+inserting non-readable tables.  By extension, this makes
+'write-abbrev-file' skip writing empty tables.
+
++++
 ** The new functions and commands 'text-property-search-forward' and
 'text-property-search-backward' have been added.  These provide an
 interface that's more like functions like @code{search-forward}.