Provide default empty StoreObject for root IDs like ROOT_QUERY.

[benjamn]

Note: this PR builds on #7098.

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

This matters because (until just recently: #7098) 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:

const cache = new InMemoryCache(); // empty
const query = gql`query { some fields }`;

// Returns null, as before:
const result = cache.readQuery({ query });

// Before PR #7098, this write would cause the ROOT_QUERY object
// to be created, changing the behavior of cache.readQuery below.
cache.writeQuery({
  query: gql`query { unrelated fields }`,
  data: {...},
});

// Previously threw a missing field error, but now returns null:
const result = cache.readQuery({
  query: gql`query { some 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.

[hwillson]

👍 LGTM - thanks @benjamn!