bug#29923: 25.3; write-abbrev-file inserts many empty tables

[Allen Li]

write-abbrev-file write a lot of empty tables.  e.g.

  (define-abbrev-table 'Buffer-menu-mode-abbrev-table '())

  (define-abbrev-table 'Info-edit-mode-abbrev-table '())

  (define-abbrev-table 'awk-mode-abbrev-table '())

  (define-abbrev-table 'bibtex-mode-abbrev-table '())

  (define-abbrev-table 'bookmark-bmenu-mode-abbrev-table '())

  (define-abbrev-table 'bookmark-edit-annotation-mode-abbrev-table '())

  (define-abbrev-table 'c++-mode-abbrev-table '())

  (define-abbrev-table 'c-mode-abbrev-table '())

It would be better (e.g., for managing abbrevs together with Emacs
configuration with version control) if empty tables were not written.

In GNU Emacs 25.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
 of 2017-12-04 built on arojas
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'