Fix NodeType config entity type config schema violations: name, description, help

Problem/Motivation

Discovered in #3364109: Configuration schema & required values: add test coverage for `nullable: true` validation support.

For example:

1) Drupal\Tests\node\Kernel\NodeTypeValidationTest::testEntityIsValid
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for node.type.kddiqdxr with the following errors: 0 [description] This value should not be null., 1 [help] This value should not be null.

Steps to reproduce

Run Drupal core's test suite with #3364109: Configuration schema & required values: add test coverage for `nullable: true` validation support applied, and with the lines

    'node.type.*' => [
      // @todo Fix config or tweak schema of `type: node.type.*`.
      // @see node.schema.yml
      'name' => [
        'This value should not be null.',
      ],
      'description' => [
        'This value should not be null.',
      ],
      'help' => [
        'This value should not be null.',
      ],
    ],

removed from \Drupal\Core\Config\Schema\SchemaCheckTrait::checkConfigSchema().

Proposed resolution

Solution:

name

Every config entity type may have a listing, so labels are essential. All node type config entities that do not have a label are currently not usable in the admin UI.

description

Either:

1. add description: '' to every single node.type.*.yml file and 'description' => '', to every single NodeType::create(…) call
2. or update \Drupal\block\Entity\NodeType::$description to default to ''.