the restore script failed when the db user has not password

[xiaojingchen]

Bug Report

What version of Kubernetes are you using?

Client Version: version.Info{Major:"1", Minor:"12", GitVersion:"v1.12.2", GitCommit:"17c77c7898218073f14c8d573582e8d2313dc740", GitTreeState:"clean", BuildDate:"2018-10-24T06:54:59Z", GoVersion:"go1.10.4", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"12", GitVersion:"v1.12.2", GitCommit:"17c77c7898218073f14c8d573582e8d2313dc740", GitTreeState:"clean", BuildDate:"2018-10-24T06:43:59Z", GoVersion:"go1.10.4", Compiler:"gc", Platform:"linux/amd64"}

What version of TiDB Operator are you using?

pingcap/tidb-operator:v1.0.0-beta.2

What did you do?

start a restore job by exec helm install --name=tidb-test-restore --namespace=tidb-test charts/tidb-backup/values.yaml

What did you expect to see?

the restore pod completed

What did you see instead?

restore pod failed, logs is

unknow flag 4000

see the restore script: https://github.com/pingcap/tidb-operator/blob/master/charts/tidb-backup/templates/scripts/_start_restore.sh.tpl#L24

when password is null, we should not use flag -p