Fix introspection error result due to missing entity type (#521)

Author: igdianov

schema/src/main/java/com/introproventures/graphql/jpa/query/schema/impl/GraphQLJpaSchemaBuilder.java

@@ -546,7 +546,8 @@ private GraphQLFieldDefinition getAggregateFieldDefinition(EntityType<?> entityT
                 var javaType = isPlural(association)
                     ? PluralAttribute.class.cast(association).getBindableJavaType()
                     : association.getJavaType();
-                var attributes = EntityIntrospector.resultOf(javaType).getAttributes();
+                var javaTypeEntity = entityManager.getMetamodel().entity(javaType);
+                var attributes = EntityIntrospector.introspect(javaTypeEntity).getAttributes();
                 var fields = attributes
                     .values()
                     .stream()