dtoc: Tidy up implementation of AddStringList()

Refactor this to avoid a loop. Also add a test for an empty string.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>