FlashScopeDistributedSessionTestCase errors due to null session reference

The getvar.xhtml file used by FlashScopeDistributedSessionTestCase invalidates a session without having done anything to create one first. That should have caused an NPE, but prior to https://github.com/eclipse-ee4j/mojarra/issues/5222 that would work because Mojarra incorrectly created a SessionScoped session instead of RequestScoped. That in turn caused blocker bug JBEAP-25420.

#5222 was fixed in Mojarra 4.0.3, and when WildFly upgraded from 4.0.2 to that, the upgrade came with a fix to getvar.xhtml to prevent the NPE. The EAP 8 branch went directly from 4.0.2 to 4.0.4 though (JBEAP-25584) and the getvar.xhtml change that went with the 4.0.3 upgrade upstream was not ported. That needs to be fixed, as now the test is continually failing.