fix bug where some requests pass variables = {} instead of variables undefined

[ian]

I'm seeing an issue with fastify + mercurius where some GraphQL playgrounds / queries are getting parsed as variables = {} instead of the expected variables = undefined

Versions:

fastify 3.25.3
mercurius 9.1.0

This is most easily seen when using GraphQL Playground vs Insomnia:

Insomnia:

before buildExecutionContext {
  document: {
    kind: 'Document',
    definitions: [ [Object] ],
    loc: { start: 0, end: 46 }
  },
  variables: undefined,
  shouldCompileJit: undefined
}

vs GQL Playground (or Altair)

before buildExecutionContext {
  document: {
    kind: 'Document',
    definitions: [ [Object], [Object], [Object], [Object] ],
    loc: { start: 0, end: 1305 }
  },
  variables: {},
  shouldCompileJit: false
}

Notice the variable values are different. There's no difference in fastify implementation, just a difference with clients.

The difference in variables value is causing mercurius to blow up on document not being set inside buildExecutionContext

[mcollina]

Can you please add a unit test?