Provide default empty StoreObject for root IDs like ROOT_QUERY.

This change means the existence of root objects like ROOT_QUERY and
ROOT_MUTATION will no longer depend on whether other queries/mutations
have previously written data into the cache.

This matters because (until just recently) cache.read would return null
for a completely missing ROOT_QUERY object, but throw missing field errors
if ROOT_QUERY existed but did not have the requested fields.

In other words, this commit hides the difference between the absence of
ROOT_QUERY and its incompleteness, so unrelated cache writes can no longer
unexpectedly influence the null-returning vs. exception-throwing behavior
of cache.read.

As an additional motivation, with AC3 field policies, it's possible now to
define synthetic root query fields that have a chance of returning useful
values even if the cache is completely empty. Providing a default empty
object for root IDs like ROOT_QUERY is important to let those read
functions be called, instead of prematurely returning null from cache.read
when ROOT_QUERY does not exist.

Note: this PR builds on PR #7098.

src/cache/inmemory/entityStore.ts

@@ -79,8 +79,18 @@ export abstract class EntityStore implements NormalizedCache {
     // should not rely on this dependency, since the contents could change
     // without the object being added or removed.
     if (dependOnExistence) this.group.depend(dataId, "__exists");
-    return hasOwn.call(this.data, dataId) ? this.data[dataId] :
-      this instanceof Layer ? this.parent.lookup(dataId, dependOnExistence) : void 0;
+
+    if (hasOwn.call(this.data, dataId)) {
+      return this.data[dataId];
+    }
+
+    if (this instanceof Layer) {
+      return this.parent.lookup(dataId, dependOnExistence);
+    }
+
+    if (this.policies.rootTypenamesById[dataId]) {
+      return Object.create(null);
+    }
   }
 
   public merge(dataId: string, incoming: StoreObject): void {