Fix #4301: Show null as "null"

Author: nicolasstucki

compiler/test-resources/repl/i4301

@@ -0,0 +1,2 @@
+scala> List(null)
+val res0: List[Null] = List(null)

library/src/dotty/Show.scala

@@ -6,7 +6,7 @@ trait Show[T] {
 
 trait LowPrioShow {
   implicit def defaultShow[T]: Show[T] = new Show[T] {
-    def show(x: T) = x.toString
+    def show(x: T) = String.valueOf(x)
   }
 }