Rework empty abbrev table omitting

There were two problems with the original implementation:

1. It changed the behavior of insert-abbrev-table-description when
READABLE is nil to sometimes insert one Emacs Lisp expression and
sometimes insert nothing.
2. It broke the tests.

This commit reworks this so that insert-abbrev-table-description
always inserts an expressions even if no abbrevs need to be saved and
making only write-abbrev-file check that a table has any abbrevs to
save before calling insert-abbrev-table-description.  This duplicates
the work of filtering the table for savable abbrevs, but the benefit
of keeping the API is worth it.

* doc/lispref/abbrevs.texi (Abbrev Tables): Update documentation.
* lisp/abbrev.el (write-abbrev-file): Skip tables without user abbrevs
(insert-abbrev-table-description): Always insert the define
expression.
(abbrev--table-symbols): New function.
* test/lisp/abbrev-tests.el (abbrev--table-symbols-test):
Add test for abbrev--table-symbols.

1 files changed, 3 insertions, 4 deletions

diff --git a/doc/lispref/abbrevs.texi b/doc/lispref/abbrevs.texi
index 4c9e653cb19..1e9471ba27a 100644
--- a/doc/lispref/abbrevs.texi
+++ b/doc/lispref/abbrevs.texi
@@ -122,9 +122,7 @@ System abbrevs are listed and identified as such.  Otherwise the
 description is a Lisp expression---a call to @code{define-abbrev-table}
 that would define @var{name} as it is currently defined, but without
 the system abbrevs.  (The mode or package using @var{name} is supposed
-to add these to @var{name} separately.)  If the Lisp expression would
-not define any abbrevs (i.e.@: it defines an empty abbrev table), this
-function inserts nothing.
+to add these to @var{name} separately.)
 @end defun
 
 @node Defining Abbrevs
@@ -234,7 +232,8 @@ Emacs commands to offer to save your abbrevs.
 Save all abbrev definitions (except system abbrevs), for all abbrev
 tables listed in @code{abbrev-table-name-list}, in the file
 @var{filename}, in the form of a Lisp program that when loaded will
-define the same abbrevs.  If @var{filename} is @code{nil} or omitted,
+define the same abbrevs.  Tables that do not have any abbrevs to save
+are omitted.  If @var{filename} is @code{nil} or omitted,
 @code{abbrev-file-name} is used.  This function returns @code{nil}.
 @end deffn