Add tests for nullification of higher kinded types

Author: HarrisL2

tests/explicit-nulls/flexible-unpickle/Flexible_2.scala

@@ -97,3 +97,7 @@ def Flexible_2() =
 
   val m: String = M.test(null)
 
+  val n1: List[Map[String, Int]] = ???
+  val n2 = N[List]()
+  val n3 = n2.accept[Any](x)
+

tests/explicit-nulls/flexible-unpickle/Unsafe_1.scala

@@ -73,3 +73,6 @@ object M {
   def test(input: => String): String = "foo " + input
 }
 
+class N[F[_]] {
+  def accept[A](arg: F[A]): Nothing = ???
+}