fix(java): guard a missing @agg before the value-object aggregate check

List.of(...).contains(null) throws, so an origin.aggregate with no @agg over a
resolvable @Of crashed the loader instead of reporting the missing attribute.

Author: dmealing

server/java/metadata/src/main/java/com/metaobjects/loader/ValidationPhase.java

@@ -3119,7 +3119,7 @@ private static void validateOriginNode(MetaRoot root, MetaObject obj,
             // Money-shaped value would add amounts in different currencies.
             // ADR-0039: resolving (hasMetaAttr defaults to includeParentData) — @objectRef may be
             // inherited via extends.
-            if (MetaOrigin.COLUMN_VALUE_AGGREGATES.contains(agg)
+            if (agg != null && MetaOrigin.COLUMN_VALUE_AGGREGATES.contains(agg)
                     && (ObjectField.SUBTYPE_OBJECT.equals(ofTarget.field.getSubType())
                         || ofTarget.field.hasMetaAttr(ObjectField.ATTR_OBJECTREF))) {
                 throw new MetaDataException(