Challenge 1: Safe Call Chains — Solution

fun main() {
    val city: String? = null
    println(city?.length)

    val realCity: String? = "London"
    println(realCity?.length)
}

/*
Output:
null
6

Notes:
- city?.length short-circuits: since city is null, the whole expression
  evaluates to null instead of throwing a NullPointerException.
- realCity?.length works through normally since realCity isn't null,
  returning the actual String.length value (6, for "London").
- Without the ?., city.length wouldn't even compile — Kotlin refuses to
  let you call a member directly on a nullable type.
*/