Fix `parentFile` is not actually nullable in NodeJS target

[y9san9]

I don't know what is the return when there is no parent file, so I decided to put here a reduntant nullability modifier:

kds/files/src/jsMain/kotlin/fun/kotlingang/kds/file/CommonFile.kt

Line 15 in f34ab7f

actual val parentFile: CommonFile? get() = CommonFile(pathModule.resolve(absolutePath, ".."))