fix small shellcheck issue
fix small shellcheck issue

  fix the following issue:
    In ./hack/verify-whitespace.sh line 28:
    if [[ ! -z "$TRAILING" ]]; then
          ^-- SC2236: Use -n instead of ! -z.

    For more information:
      https://www.shellcheck.net/wiki/SC2236 -- Use -n instead of ! -z.

Signed-off-by: Ma Xinjian <d1eaa362fbe20f4d1e94a590b52fa846afcd0e34@cn.fujitsu.com>

kubernetes/kubeadm,kubernetes/kubeadm