Make NextConfigComplete typing more accurate (#96700)
These properties were previously typed as always set in
`NextConfigComplete`. But in reality, they can be optional.
```
| 'expireTime'
| 'output'
| 'modularizeImports'
| 'allowedDevOrigins'
| 'adapterPath'
```
I couldn't find a better way to fix this than listing them out
explicitly
