Turbopack: also inline env vars with undefined value (#80990)
Previously, only env vars with set values were inline.
But some of them, e.g. `process.env.__NEXT_MANUAL_TRAILING_SLASH` and `process.env.__NEXT_CROSS_ORIGIN` are always set, but might be `undefined` in the env mapping.
Don't drop these but replace them with undefined, as Webpack does.
