I think the cause of the problem is that encode_codin_ccl() does not
call ccl_driver() for zero length string.  Please see the attached
patch.

> When I start emacs (emacs-23 branch and trunk) with -Q and evaluate
> below code, I receive unexpected result.
> 
> (let ((string ""))
>   (define-ccl-program ccl-test
>     '(1
>       ((read r0))
>       ((write "[EOF]"))))
>   (make-coding-system 'ccl-test-cs 4 ?T "CCL_EOF_BLOCK tester"
> 		      '(ccl-test . ccl-test))
>   (list
>    (ccl-execute-on-string 'ccl-test (make-vector 9 0) string)
>    (decode-coding-string string 'ccl-test-cs)
>    (encode-coding-string string 'ccl-test-cs)))
> 
> It returns
>   ("[EOF]" "[EOF]" "")
> But emacs22 returns
>   ("[EOF]" "[EOF]" "[EOF]")
> and I expect such result.

-- 
Kazuhiro Ito