Add tests for nullification of higher kinded types

Author: HarrisL2

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

@@ -100,6 +100,11 @@ def Flexible_2() =
 
   val m: String = M.test(null)
 
+  // i23911
+  val n1: List[Map[String, Int]] = ???
+  val n2 = N[List]()
+  val n3 = n2.accept[Any](x)
+
   // i23845
   transparent inline def typeName[A]: String = ${typeNameMacro[A]}
 
@@ -109,3 +114,4 @@ def Flexible_2() =
     implicit val givenS: S[A] = ???
     expects(alphaTypeNameMacro[A])
   }
+

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

@@ -73,6 +73,11 @@ object M {
   def test(input: => String): String = "foo " + input
 }
 
+
+class N[F[_]] {
+  def accept[A](arg: F[A]): Nothing = ???
+}
+
 class S[X]
 object S {  def show[X] = "dummyStr" }
 class T