publicRuntimeConfig shouldn’t be undefined on build

Bug report

Describe the bug

v8.1.1-canary.62 (or maybe 3aed76fad857b518a31675987e8c79f6f1afccde) (re-)introduces an issue with how I use (and recommend how to use) https://github.com/tusbar/next-runtime-dotenv.

To Reproduce

This breaks on build as publicRuntimeConfig is undefined at build time:

const {
  publicRuntimeConfig: {MY_API_URL}
} = getConfig()

Expected behavior

publicRuntimeConfig used to be {} at build time.

System information

• OS: macOS
• Version of Next.js: 8.1.1-canary.62

Additional context

This is not the first time this breaks, maybe it would be nice to add a test. It used to work fine with 8.0.4, it’s broken in 8.1.0 and used to work fine in canary before 8.1.1-canary.62. Broken in 9.0 as well.

[jtaox]

I reproduced this bug on 9.4.2

[Multiply]

I’ve tried v9.5.2 which should include #15777 and we’re still getting undefined back from any getConfig call it seems.