What does lookup("known") print when the receiver exists but the let block returns null?

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"))
}