Cannot return null for non-nullable field ServiceMutation.upsertImplementingServiceAndTriggerComposition

**Intended outcome:** For `apollo service:push` to successfully and deterministically push the updated schema.

**Actual outcome:** Roughly 50% of the time, `apollo service:push` throws a `Validation Error`: `Cannot return null for non-nullable field ServiceMutation.upsertImplementingServiceAndTriggerComposition`. If we run the command again while making NO changes, there is a 50% chance it succeeds.

```
> apollo service:push --variant development --serviceName serviceName --serviceURL http://localhost:4200/graphql --localSchemaFile /root/project/path/to/schema.gql
Loading Apollo Project
✔ Loading Apollo Project
⠼ Uploading service to Apollo
✔ Loading Apollo Project
⠇ Uploading service to Apollo
✔ Loading Apollo Project
⠴ Uploading service to Apollo
✔ Loading Apollo Project
⠏ Uploading service to Apollo
[GraphQL error]: Validation error
[GraphQL error]: Cannot return null for non-nullable field ServiceMutation.upsertImplementingServiceAndTriggerComposition.
✔ Loading Apollo Project
✖ Uploading service to Apollo

Error: Validation error
Cannot return null for non-nullable field
ServiceMutation.upsertImplementingServiceAndTriggerComposition
```

**How to reproduce the issue:** It has been very inconsistent, sometimes happening two or three times in a row before succeeding.

**Versions**
```
> apollo --version
apollo/2.33.4 darwin-x64 node-v12.13.1
```

Start by reproducing the intermittent failure with apollo/2.33.4 using the documented apollo service:push command and the supplied variant, service name, URL, and local schema file. Compare successful and failing uploads around the ServiceMutation.upsertImplementingServiceAndTriggerComposition validation error. Done means the command pushes the updated schema successfully and deterministically.

Written by the indexing model from the issue text.