correctly generate safe casts from Unit type

#KT-4494 fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/ExpressionCodegen.java

@@ -3840,6 +3840,9 @@ The "returned" value of try expression with no finally is either the last expres
                                 v.mark(nonnull);
                             }
                         }
+                        else if (value.type == Type.VOID_TYPE) {
+                            v.aconst(null);
+                        }
                         else {
                             v.dup();
                             generateInstanceOfInstruction(rightType);

compiler/testData/codegen/box/casts/unitNullableCast.kt

@@ -0,0 +1,7 @@
+fun foo() {}
+
+fun bar(): Int? = foo() as? Int
+
+fun box(): String {
+    return if (bar() == null) "OK" else "fail"
+}

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -930,6 +930,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/casts/notIs.kt");
             doTest(fileName);
         }
+
+        @TestMetadata("unitNullableCast.kt")
+        public void testUnitNullableCast() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/casts/unitNullableCast.kt");
+            doTest(fileName);
+        }
     }
 
     @TestMetadata("compiler/testData/codegen/box/classes")