Get migration plugin returns null when source is array

Done.

I found this issue trying to find why the processors with handle_multiple false, with multiple sources where receiving an array, when processRow is suposed to handle that, and it was because of the Get processor setting multiple from FALSE to NULL in MigrateExecutable->processRow()

I was importing a product entity and I had this processor to fill a field with products of the same kind but different color:

field_also_available:
    plugin: migration_lookup
    migration: migrate_cases_csv
    source:
      - color_available_1
      - color_available_2

The current migration is migrate_cases_csv.

As migration_lookup has handle_multiple to FALSE (default) and field_also_available is a multivalued field, I expect the processor to iterate the sources in MigrateExecutable->processRow() and fill a new entry for each source, but as in reality this executes first the Get plugin and then the migration_lookup plugin, In this fragment of code inside processRow():

      $multiple = FALSE;
      /** @var $plugin \Drupal\migrate\Plugin\MigrateProcessInterface */
      foreach ($plugins as $plugin) {
        $definition = $plugin->getPluginDefinition();
        // Many plugins expect a scalar value but the current value of the
        // pipeline might be multiple scalars (this is set by the previous
        // plugin) and in this case the current value needs to be iterated
        // and each scalar separately transformed.
        if ($multiple && !$definition['handle_multiples']) {

Get in the first iteration is setting multiple to NULL when it is returning an array, so should set it to TRUE.
But setting it to null acts like with FALSE, so it gives the value to migrate_lookup without iterate over it, and migrate_lookup behaves unexpectedly as it just returns the first that it finds.

The failing tests are because there are other process plugins that should have handle_multiples TRUE, as the can do it, but they where relying in this wrong behavior of Get.
One example that I'm fixing here is FileCopy, that handles multiples as expects an array of source and destination, but did not have handle_multiples.

This is not working.
As multiple by default is FALSE even with array, because of that, Get gets called just once, but as Get does not care of value but uses the config property source, it gets the array and returns it, so there is no actual difference between
this:

field_also_available:
    -
      plugin: get
      source:
        - color_available_1
        - color_available_2
    -
      plugin: migration_lookup
      migration: migrate_cases_csv

and this:

field_also_available:
    plugin: migration_lookup
    migration: migrate_cases_csv
    source:
      - color_available_1
      - color_available_2

I still think this is a bug in Drupal as the behavior of handle_multiple does not match the documentation https://api.drupal.org/api/drupal/core%21modules%21migrate%21src%21Annotation%21MigrateProcessPlugin.php/property/MigrateProcessPlugin%3A%3Ahandle_multiples/8.3.x

I still think that this is a bug as the MigrateProcessPlugin behaves different from the documentation, so we have to change documentation or the plug-in.
And as I said the response from 9 is not working.