isNullOrEmpty

import kotlin.test.*

fun main() { 
   //sampleStart 
   fun markdownLink(title: String?, url: String) =
    if (title.isNullOrEmpty()) url else "[$title]($url)"

// plain link
println(markdownLink(title = null, url = "https://kotlinlang.org")) // https://kotlinlang.org

// link with custom title
println(markdownLink(title = "Kotlin Language", url = "https://kotlinlang.org")) // [Kotlin Language](https://kotlinlang.org) 
   //sampleEnd
}