[BISERVER-11536][REGRESSION]Unable to add new roles

Null pointer exception was thrown when adding a new role via UserRoleWebService.
Fix by creating role with empty user list instead of null.

http://jira.pentaho.com/browse/BISERVER-11536
Merge pull request #1619 from hudak/BISERVER-11536

[BISERVER-11536][regression]Unable to add new roles