Fix NonNull test

The test case `tests/explicit-nulls/neg/strip.scala` specify that null
unions inside intersection types should work.  After changing the
scrutinee type of the extractor `OrNull` it is no longer the case.

Changing the scrutinee type is still justified because it agrees with
the name as well as the usage in `Types.scala`.

In contrast, in `Typer.scala`, the logic is more clear without using `OrNull`.

Author: liufengyun

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -477,15 +477,17 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
    */
   def toNotNullTermRef(tree: Tree, pt: Type)(using Context): Tree = tree.tpe match
     case ref: TermRef
-    if pt != AssignProto && // Ensure it is not the lhs of Assign
+    if ctx.explicitNulls &&
+    pt != AssignProto && // Ensure it is not the lhs of Assign
     ctx.notNullInfos.impliesNotNull(ref) &&
     // If a reference is in the context, it is already trackable at the point we add it.
     // Hence, we don't use isTracked in the next line, because checking use out of order is enough.
     !ref.usedOutOfOrder =>
-      ref.widenDealias match
-      case OrNull(tpnn) =>
-        tree.cast(AndType(ref, tpnn))
-      case _ =>
+      val tp1 = ref.widenDealias
+      val tp2 = tp1.stripNull()
+      if tp1 ne tp2 then
+        tree.cast(AndType(ref, tp2))
+      else
         tree
     case _ =>
       tree