hush: return consistent codes from run_command()

Attempting to run:
 - an empty string
 - a string with just spaces

returns different error codes, 1 for the empty string and 0
for the string with just spaces.  Make both of them return
0 for consistency.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Simon Glass <sjg@chromium.org)>