"SERVICE_DOMAIN" may be null while Zeppelin server is running outside k8s

The function `sparkUiWebUrlFromTemplate` needs to use the variable "SERVICE_DOMAIN", which is assigned to the default value of system environment variable "SERVICE_DOMAIN".

However, user may not set the environment variable when Zeppelin server is running outside of k8s, and it will get a NullPointerExcepetion:

NullPointerException: null value in entry: SERVICE_DOMAIN=null

Maybe it is better to set a non-null default value for it.