People tend to forget about extension functions. What do you think about these alternatives:

fun Any?.isNull() = this == null

and

fun Any?.isNotNull() = this != null

. That way you can assert any nullable type in a more readable way (the Kotlin way IMHO)

val a: String? = null
val b: String? = ""

if(a.isNull() && b.isNotNull()) {
 ...
}

You can also add contracts to enable smart casts:

fun Any?.isNull(): Boolean {
 contract {
        returns(true) implies (this == null)
    }
 return this == null
}

*Note: contracts are experimental.