serialize to relative url only when id does not exist

set null only when card.id does not exist

packages/base/card-api.gts

@@ -697,10 +697,12 @@ class Contains<CardT extends FieldDefConstructor> implements Field<CardT, any> {
   serialize(
     value: InstanceType<CardT>,
     doc: JSONAPISingleResourceDocument,
+    _visited: Set<string>,
+    opts?: SerializeOpts,
   ): JSONAPIResource {
     let serialized: JSONAPISingleResourceDocument['data'] & {
       meta: Record<string, any>;
-    } = callSerializeHook(this.card, value, doc);
+    } = callSerializeHook(this.card, value, doc, undefined, opts);
     if (primitive in this.card) {
       return { attributes: { [this.name]: serialized } };
     } else {

packages/host/app/services/card-service.ts

@@ -213,8 +213,9 @@ export default class CardService extends Service {
         // for a brand new card that has no id yet, we don't know what we are
         // relativeTo because its up to the realm server to assign us an ID, so
         // URL's should be absolute
-        maybeRelativeURL: null, // forces URL's to be absolute.
+        maybeRelativeURL: card.id ? undefined : null, // forces URL's to be absolute when no ID
       });
+
       // send doc over the wire with absolute URL's. The realm server will convert
       // to relative URL's as it serializes the cards
       let realmURL = await this.getRealmURL(card);