Fix hush to give the correct return code for a simple command

When a simple command like 'false' is provided, hush should return the
result of that command. However, hush only does this if the
FLAG_EXIT_FROM_LOOP flag is provided. Without this flag, hush will
happily execute the empty string command immediate after 'false' and
then return a success code.

This behaviour does not seem very useful, and requiring the flag also
seems wrong, since it means that hush will execute only the first command
in a sequence.

Add a check for empty string and fall out of the loop in that case. That
at least fixes the simple command case. This is a change in behaviour but
it is unlikely that the old behaviour would be considered correct in any
case.

Reported-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
Signed-off-by: Simon Glass <sjg@chromium.org>