接收者存在但 let 块返回 null 时，lookup("known") 会输出什么？

fun source(key: String): String? =
    if (key == "known") "   " else null

fun lookup(key: String): String =
    source(key)?.let { value ->
        value.trim().takeIf { it.isNotEmpty() }
    } ?: "missing"

fun main() {
    println(lookup("known"))
}