Multiple Group Bulk Export rules conflict

[tadgh]

The current implementation of the RuleBulkExportImpl returns a DENY if the group ID does not match, but it is possible for another rule to match. E.g. if we were to write the following rules:

				return new RuleBuilder()
					.allow().bulkExport().groupExportOnGroup(new IdType("Group/123")).andThen()
					.allow().bulkExport().groupExportOnGroup(new IdType("Group/789")).andThen()
					.build();

This would cause a failure for somebody attempting to export Group/789, as first the Group/123 rule is hit, and it returns a deny. If instead it returns null, it can delegate to further rules.