Toggle follow partner doesn't always purge GraphCDN

[brettski]

When someone toggles to follow a partner, the cache for that partner (by id) is only purged when the user is added as a follower, not removed.

A basic toggle mutation is:

mutation togglePartnerFavorite {
  partners {
    favoriting(findBy: {slug: "google-cloud"}) {
      toggle {
	__typename
        id
        companyName
        slug
      }
    }
  }
}

The JSON Result is either:

{
	"data": {
		"partners": {
			"favoriting": {
				"toggle": {
					"__typename": "Partner",
					"id": "a9uFn5IBuCLmTfar7wLF",
					"companyName": "Google Cloud",
					"slug": "google-cloud"
				}
			}
		}
	}
}

or

{
	"data": {
		"partners": {
			"favoriting": {
				"toggle": null
			}
		}
	}
}

As you can see by the second result the response is NULL and will not trigger a purge at GraphCDN.

At this point I see two options to correct this.

a. Add a purging event to the partners API and trigger a purge when the toggle mutation is called
b. Refactor the response from the mutation so a value is always returned, though an additional field may be required to let the caller know if they are now following or not following the partner (currently known by the null/not null response).