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

@@ -53,4 +53,16 @@ public final class EqualsTest extends AbstractExpressionTest {
     public void testCompareNullableListWithNull() throws Exception {
         checkFooBoxIsOk();
     }
+
+    public void testCompareToNullWithCustomEquals() throws Exception {
+        checkFooBoxIsOk();
+    }
+
+    public void testCompareNullablesWithCustomEquals() throws Exception {
+        checkFooBoxIsOk();
+    }
+
+    public void testEqualsBehaviorOnNull() throws Exception {
+        checkFooBoxIsOk();
+    }
 }