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/etc/NEWS b/etc/NEWS
index ee74e86f40f..946a823173a 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -258,10 +258,9 @@ 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.
+** 'write-abbrev-file' skips empty tables.
+'write-abbrev-file' now skips inserting a 'define-abbrev-table' form for
+tables which do not have any non-system abbrevs to save.
 
 +++
 ** The new functions and commands 'text-property-search-forward' and