Disallow new Array[Null,Nothing] or allow classTag[Null,Nothing].

[sjrd]

We need to either disallow new Array[Null](0) and new Array[Nothing](0), or allow the materialization of ClassTag[Null] and ClassTag[Nothing]. Otherwise, things are inconsistent.

For example, currently we have the following:

object Test {
  def main(args: Array[String]): Unit = {
    println(new Array[Null](0)) // ok
    println(new Array[Nothing](0)) // ok

    println(scala.reflect.classTag[Null]) // No ClassTag available for Null
    println(scala.reflect.classTag[Nothing]) // No ClassTag available for Nothing
  }
}

Either all 4 statements should succeed, or all of them should fail to compile.

For context, disallowing ClassTag[Null] and ClassTag[Nothing] came from #1730 based on the rationale of avoiding a specific kind of ClassKindException. The example from that issue can still be reproduced today using new Array:

object Test {
  def main(args: Array[String]): Unit = {
    val x = if (false) new Array[String](0) else new Array[Nothing](0)
    val y: Array[_ <: String] = x
    println(y)
  }
}

results in

Exception in thread "main" java.lang.ClassCastException: [Lscala.runtime.Nothing$; cannot be cast to [Ljava.lang.String;
        at Test$.main(hello.scala:3)
        at Test.main(hello.scala)