Missing null checks in auth client lead to NPEs later on

There are missing null checks in at least the following places:

• The constructor of org.wildfly.security.auth.principal.NamePrincipal which should require "name" to be non-null
• The name argument to org.wildfly.security.auth.client.AuthenticationConfiguration#useName

In addition, the following places handle null wrongly:

• org.wildfly.security.auth.client.AuthenticationConfiguration#useAuthorizationName should check name for null, and if it is null, pass null in to useAuthorizationPrincipal