Fix coprocessor empty body panic

[BrynCooke]

If a coprocessor responds with an empty body at the supergraph stage then the router would panic as there was a false expectation that the query had already been validated.

There is a deeper issue around coprocessors and their ability to modify the query before processing. As query analysis and plugins may use the query to populate context modifying the query won't necessarily do what the user wants. In fact the router will simply ignore the modified query as the parsed version is already in context. But that will be tackled in future pipeline rework.

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

• No unit tests as it is the interaction across the entire pipeline that highlights the issue. Instead extensive integration tests have been added.
• No docs change as this is a bug fix.

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

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