Require placeholder resources for non-proxy runtime resources

ReadResourceHandler has a nullSafeReadResource method that returns a PlaceholderResource for any address whose MRR says it's runtime:

https://github.com/wildfly/wildfly-core/blob/master/controller/src/main/java/org/jboss/as/controller/operations/global/ReadResourceHandler.java#L435

That basically allows devs to not bother creating runtime resources, but breaks things where there's an assumption that no resource object will be present when no actual resource exists. See discussion at https://github.com/wildfly/wildfly/pull/6607

This JIRA is to get rid of that behavior, restricting it to, at most, proxy resource cases.

See also WFCORE-580 which is a similar problem.