ManagementResourceRegistration.getOverrideModel never returns null

ManagementResourceRegistration.getOverrideModel ends up returning the wildcard registration if there is no override registration. This isn't correct.

The fix isn't trivial because fixing it results in nasty failures in the smoke tests. From looking at the uses of this method (which all involve a null check) I assume there are some bugs in the code that calls this method that get exposed once it does what it should.

This bug is the cause of the initial failure of my WFLY-2880 fix.