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)

test/command_ut.c

@@ -188,6 +188,9 @@ static int do_ut_cmd(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 #endif
 #endif
 
+	assert(run_command("", 0) == 0);
+	assert(run_command(" ", 0) == 0);
+
 	printf("%s: Everything went swimmingly\n", __func__);
 	return 0;
 }