JS_IR: fix typecheck corner case

Consider `fun <E : I> foo(a: Any?) = a as? E`, where I is an interface.
This check used to fail, because the `a == null` was missing, and
the `isInterface` stdlib method crashes if the first argument
is null. This change adds the null check.

Also this change prettifies the instance check in case of type parameter
left operand.

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

@@ -2821,6 +2821,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/casts/notIs.kt");
         }
 
+        @TestMetadata("nullableSafeCastToTypeParameterWithInterfaceUpperBound.kt")
+        public void testNullableSafeCastToTypeParameterWithInterfaceUpperBound() throws Exception {
+            runTest("compiler/testData/codegen/box/casts/nullableSafeCastToTypeParameterWithInterfaceUpperBound.kt");
+        }
+
         @TestMetadata("unitAsAny.kt")
         public void testUnitAsAny() throws Exception {
             runTest("compiler/testData/codegen/box/casts/unitAsAny.kt");