mode_flag: make it empty array

Since bash-4.4, "${array[@]:+${array[@]}}" resolves to "", whereas
before it was resolving to an empty array.
_check_mount_image_flags expects an empty array, make it so.

BUG=None
TEST=unit tests

Change-Id: I9de8439bcd8e20098b162c21dd03b6adcc3ac856
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2685277
Commit-Queue: Sergey Frolov <sfrolov@google.com>
Tested-by: Sergey Frolov <sfrolov@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed