fix: handle undefined reply object inside mercurius context

[DavideArena]

There's a scenario where "reply" object inside mercurius context is undefined. In this case, the plugin crashes whit the following error:

{ "error": "Internal Server Error", "message": "Cannot read properties of undefined (reading 'request')", "statusCode": 500 }

We found this problem in our use case:

• Create a fastify GET endpoint, executing internally the graphql mercurius federation query for sdl { _service { sdl } }.
• Create a test for new endpoint, triggering request via fastify.inject.

In this PR i added a guard, checking if context.reply is not undefined. In case of undefined, the plugin doesn't make anything.

[Eomm]

Nice catch!

It would be helpful to mention this usage in the documentation:

If the [mercurius `graphql` decorator](https://github.com/mercurius-js/mercurius/blob/master/docs/api/options.md#appgraphqlsource-context-variables-operationname) is used, it is necessary to provide a `context` object: `app.graphql(query, { reply })`.
Otherwise, this plugin will ignore the request.

[DavideArena]

@Eomm updated documentation with mention and added the test.

[Eomm]

Grazie Davide!