Add set_on_condition process plugin

Problem/Motivation

Is this a good idea?

<?php

namespace Drupal\migrate_conditions\Plugin\migrate\process;

use Drupal\Component\Plugin\PluginManagerInterface;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;
use Drupal\migrate_conditions\Plugin\ProcessPluginWithConditionBase;

/**
 * Sets destination property with source value if condition is met.
 *
 * This is most useful when re-migrating content. It is part of a possible
 * solution to the problem of how to avoid overwriting existing data
 *
 * Available configuration keys:
 * - dest_id: The destination id to set with the source value.
 * - condition: The condition plugin to evaluate on each element.
 *   Can be either:
 *   - The id of the condition. This is possible if the condition does not
 *     require any configuration, such as the 'empty' condition.
 *   - An array with a 'plugin' key that is the id of the condition.
 *     Any additional properties will be used as configuration when
 *     creating an instance of the condition.
 *
 * Examples:
 *
 * Don't overwrite existing values on field_my_field. Write my_source_value
 * to field_foo as long as field_foo is not already set. We assume that you
 * have somehow found the nid into which you are migrating, possibly using
 * a migrate_lookup or entity_lookup or possibly because it's in the source.
 *
 * @code
 * process:
 *   _field_foo:
 *     plugin: entity_value
 *     source: nid_into_which_we_are_migrating
 *     entity_type: node
 *     field_name: field_foo
 *   _set_field_foo:
 *     plugin: set_on_condition
 *     dest_id: field_foo
 *     source: my_source_value
 *     condition:
 *       plugin: empty
 *       source: '@_field_foo'
 * @endcode
 *
 * This is different from using overwrite_properties since (a) we don't have
 * to maintain a list of overwrite_properties if the migration is edited and
 * (b) the logic depends on the current value of the field rather than solely
 * whether the node is new.
 *
 * Skip the row is the source_value is not empty.
 * @MigrateProcessPlugin(
 *   id = "set_on_condition",
 *   handle_multiples = TRUE
 * )
 */
class SetOnCondition extends ProcessPluginWithConditionBase {

  /**
   * Constructs a SetOnCondition object.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Component\Plugin\PluginManagerInterface $condition_manager
   *   The MigrateCondition plugin manager.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, PluginManagerInterface $condition_manager) {
    parent::__construct($configuration, $plugin_id, $plugin_definition, $condition_manager);
    if (!isset($configuration['dest_id'])) {
      throw new \InvalidArgumentException('The "dest_id" must be configured.');
    }
  }

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if ($this->condition->evaluate($value, $row)) {
      $row->setDestinationProperty($this->configuration['dest_id'], $value);
      return TRUE;
    }
    return FALSE;
  }

}