JS backend: fix equality check for nullable types

* Generate simple check when compare with `null` literal.
* Use Kotlin.equals if type of receiver or argument is nullable.
* Don't call equals method if any of receiver or argument is null (at runtime, not only literal). For more information see KT-4356.

 #KT-7530 fixed
 #KT-7916 fixed

js/js.translator/testData/kotlin_lib.js

@@ -47,6 +47,10 @@
             return obj2 == null;
         }
 
+        if (obj2 == null) {
+            return false;
+        }
+
         if (Array.isArray(obj1)) {
             return Kotlin.arrayEquals(obj1, obj2);
         }