8156142: ModuleReader instances don't always throw NPE for passed null args

[Alan Bateman]

Another trivial patch, this time it's the ModuleReader for the system 
modules is missing a null check so that it doesn't throw the expected 
NPE. The patch is to mostly just expand test coverage.
   http://cr.openjdk.java.net/~alanb/8156142/webrev/

Thanks,
Alan