fix: undefined app.server for next 13.4

[alexandrucancescu]

fastify-nextjs is not working for next@13.4 because it breaks when trying to access

const basePath = app.server.nextConfig.basePath || ''

with Cannot read properties of undefined (reading 'nextConfig')

To recreate this issue, update next to the latest version and run the tests, all fail with this error.

NextServer provides the getServer() method which returns a promise that returns the server. I changed it to use this function instead of app.server and fortunately this is also compatible with 13.0.0, the version the tests were previously running on.

Tests are now passing with 13.0 and 13.4

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[Uzlopak]

It seems that nextjs is not supporting node 14 anymore. Can you check that please?

[mcollina]

@Uzlopak PTAL?

[Songkeys]

Hi, when will this cut a new release? 👀

[Eomm]

Done