Support 1-N with subclass-table and compound identity, or trap in metadata processing

[andyjefferson]

I found a small issue with certain metadata files and throwing a java.lang.NullPointerException in org.datanucleus.store.rdbms.table.ClassTable#1046 where the datastoreClass variable may be null, and failing during the test datastoreClass.getIdMapping().

Please find attached to this issue a patch to apply to give the necessary information in the log and not the plain java.lang.NullPointerException.