Make nullable check less verbose in JS-backend.

For check what variable is equal null or undefined enough to compare with null (or undefined).
See ECMAScript specification 11.9.1, 11.9.2, 11.9.3

js/js.tests/test/org/jetbrains/k2js/test/semantics/EqualsTest.java

@@ -45,4 +45,8 @@ public final class EqualsTest extends AbstractExpressionTest {
     public void testKt2370() throws Exception {
         fooBoxTest();
     }
+
+    public void testEqualsNullOrUndefined() throws Exception {
+        checkFooBoxIsOk();
+    }
 }