Fix NPE for any type of show

Author: nicolasstucki

compiler/test-resources/repl/i4301

@@ -1,2 +1,4 @@
 scala> List(null)
 val res0: List[Null] = List(null)
+scala> List[String](null)
+val res1: List[String] = 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) = if (x == null) "null" else x.toString
+    def show(x: T) = x.toString
   }
 }
 
@@ -16,8 +16,7 @@ object Show extends LowPrioShow {
     * any `T`, we default to `T#toString`.
     */
   implicit class ShowValue[V](val v: V) extends AnyVal {
-    def show(implicit ev: Show[V]): String =
-      ev.show(v)
+    def show(implicit ev: Show[V]): String = if (v == null) "null" else ev.show(v)
   }
 
   /** Adds escaping backslashes in a string so that it could be put in source code.