discover/grub: Fix handling of empty strings

If "" or '' are used in a statement to omit a word, we must still
return a TOKEN_WORD for an empty string.

In particular this fixes an issue where Petitboot would fail to parse
the grub.cfg included in the Debian 8.2 install image, which includes a
menuentry statement with an empty name.

Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>