unable to remove previously deleted role form default roles

If you try to remove an user from default roles and this user was previously deleted you will get a stack trace.
```
An error occurred whilst rendering this message. Please contact the administrators, and inform them of this bug. Details: ------- org.apache.velocity.exception.MethodInvocationException: Invocation of method 'getUserPickerHtml' in class com.atlassian.jira.web.action.admin.roles.UserRoleActorAction threw exception java.lang.NullPointerException at com.atlassian.jira.plugin.system.projectroleactors:atlassian-user-role-actor-action//templates/plugins/jira/projectroles/user-role-actor.vm[line 21, column 9] at org.apache.velocity.runtime.parser.node.ASTIdentifier.execute(ASTIdentifier.java:223) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:262) at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:342) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:336) at org.apache.velocity.Template.merge(Template.java:328) at or
```

This can happen especially if you have external directories where users can be removed at any time.