Issue #2827897 by quietone, danflanagan8, mikelutz, smustgrave, mikeryan,...

 * - default_value: (optional) The value to return if the source is not found in

 *   the map array.

 *

 * While this plugin supports map key values which contain a dot (.), Drupal

 * configuration export does not export keys which contain a dot. Be careful

 * when using this feature with migrations that are stored as configuration

 * entities. These entities cannot contain keys with a dot. In this case,

 * additional manipulation with either custom or contrib process plugins is

 * needed.

 *

 * Examples:

 *

 * If the value of the source property 'foo' is 'from' then the value of the

 *       1: bar

 * @endcode

 *

 * Mapping from a string which contains a period is not supported. A custom

 * process plugin can be written to handle this kind of a transformation.

 * Another option which may be feasible in certain use cases is to first pass

 * the value through the machine_name process plugin.

 *

 * @see https://www.drupal.org/project/drupal/issues/2827897

 * @see \Drupal\migrate\Plugin\MigrateProcessInterface

 */

    $this->assertSame('mapped NULL', $value);

  }

  /**

   * Tests when there is a dot in a map key.

   */

  public function testMapDotInKey(): void {

    $configuration['map']['foo.bar'] = 'baz';

    $this->plugin = new StaticMap($configuration, 'map', []);

    $value = $this->plugin->transform('foo.bar', $this->migrateExecutable, $this->row, 'destination_property');

    $this->assertSame('baz', $value);

  }

}