wrap null qname deref in xsb group loaders

[aizu-m]

Traced why a corrupt .xsb drops out of the loader as a bare NullPointerException rather than the usual SchemaTypeLoaderException.

1. readQName returns null when the local-part string code is 0, because stringForCode(0) is null.
2. finishLoadingModelGroup and finishLoadingAttributeGroup read the QName and dereference name.getNamespaceURI() before entering their try, so that null escapes as a raw NPE out of resolveHandle.
3. the sibling loaders (finishLoadingElement, finishLoadingAttribute, finishLoadingIdentityConstraint) already do the same readQName/getContainer work inside the try, so they report SchemaTypeLoaderException for the identical input.

Moved the readQName/getContainer/checkContainerNotNull lines into the try in both group loaders so a null name surfaces the same way as everywhere else. Added a test that feeds a zero local-part code through both loaders and expects SchemaTypeLoaderException.