Revert "Issue #1813832 by andypost: Fixed Allow use '0' as id for entity."

This reverts commit f8673ac4.

Needs more discussion.

core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php

@@ -51,8 +51,7 @@ public function getOriginalID() {
    * since each configuration entity is unique.
    */
   final public function isNew() {
-    $id = $this->id();
-    return !isset($id);
+    return !$this->id();
   }
 
   /**

core/lib/Drupal/Core/Entity/Entity.php

@@ -91,8 +91,7 @@ public function uuid() {
    * Implements EntityInterface::isNew().
    */
   public function isNew() {
-    $id = $this->id();
-    return !empty($this->enforceIsNew) || !isset($id);
+    return !empty($this->enforceIsNew) || !$this->id();
   }
 
   /**

core/modules/config/lib/Drupal/config/Tests/ConfigEntityTest.php

@@ -78,16 +78,13 @@ function testCRUD() {
     // Rename the configuration entity's ID/machine name.
     $this->assertLinkByHref('admin/structure/config_test/manage/' . $id);
     $edit = array(
-      'id' => '0',
+      'id' => strtolower($this->randomName()),
       'label' => $label3,
     );
     $this->drupalPost('admin/structure/config_test/manage/' . $id, $edit, 'Save');
     $this->assertResponse(200);
     $this->assertNoText($label1);
     $this->assertText($label3);
-    $this->drupalPost('admin/structure/config_test/manage/0/delete', array(), 'Delete');
-    $this->assertResponse(200);
-    $this->assertNoText($label3);
   }
 
 }