In GRUB2 parser save_env, treat unset variable value as empty

In GRUB2 parser save_env, treat unset variable value as empty

It seems better to treat unset variable values as empty rather than
crashing in save_env.  While GRUB's behavior is actually to delete the
variable from the environment block, it seems useful to at least not
crash while later on someone can do further work to improve GRUB
compatibility if desired.

Tested:
 Modified test-grub2-save-env to cover this case.

Signed-off-by: Alan Dunn <amdunn@google.com>
Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>