Do not report USELESS_CAST when casting null to nullable (special case)

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/BasicExpressionTypingVisitor.java

@@ -331,6 +331,7 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
     }
 
     private static boolean isUpcast(KotlinType candidateType, KotlinType targetType, KotlinTypeChecker typeChecker) {
+        if (KotlinBuiltIns.isNullableNothing(candidateType)) return false;
         if (!typeChecker.isSubtypeOf(candidateType, targetType)) return false;
 
         if (isFunctionType(candidateType) && isFunctionType(targetType)) {

compiler/testData/diagnostics/tests/cast/NullableToNullable.kt

@@ -1,5 +1,5 @@
 // From KT-13324: always succeeds
-val x = null <!USELESS_CAST!>as String?<!>
+val x = null as String?
 // From KT-260: sometimes succeeds
 fun foo(a: String?): Int? {
     val c = a as? Int?