Allow Rx Single to map to nullable values by converting to Maybe. #kotlin

RxExt.kt

/**
 * Maps the [Single] value [T] to a nullable type [R?] which will become [Maybe.empty] when `R == null`,
 * otherwise [Maybe.just]
 */
inline fun <T, R> Single<T>.mapNullable(crossinline mapper: (T) -> R?): Maybe<R> = this.flatMapMaybe {
    val result = mapper(it)
    if (result == null) Maybe.empty() else Maybe.just(result)
}