Default Value Migration Process plugin produces unexpected behavior when updating list field from D7 to D8

Scenario

Migrating a D7 list text field to a D8 list text field. The field is a select field so only one value can be selected. Some of the D7 list fields values were never set (it was not required). In D8, the field is now required so I want to set the default value during the migration for any of those fields that never had a value on the D7 site.

The list text field has three possible values ("top", "bottom" and "center") and we want "center" to be the default if nothing was ever set on the D7 site. Here is the migration config for the process plugin.

field_hero_image_position:
  plugin: default_value
  default_value: center
  source: field_banner_image_position

Expected Behavior

The expected behavior is that the migration will migrate over the content from the D7 field to the D8 field if it is set. However, in the event that it was not set, the default_value plugin should be run in which case it would determine the value to be empty and therefore use center as the default value.

How to Reproduce

Migrate a D7 list text field with unset values (I am not sure if this applies to any other field type) to a D8 list text field and try to apply a default value during the migration.

If you remove the source line from the migration configuration. The default_value plugin works as expected and the default is set on the field.

Why it's happening (see code snippets below)

1. MigrateExecutable.php calls the processRow method for each row. This loops through each field and plugin. In this case we have one field but it sets the field as having two plugins (Get and DefaultValue).
1. The Get process plugin runs first, and tries to grab the value but there is no value set, so it sets $value to an empty array.
2. The process then sets the $multiple variable to TRUE. This is important because it controls what happens in the next iteration of the loop.
3. On the second iteration of the loop, the code falls into the first if statement since $multiple variable is now TRUE.
4. Since $value is an empty array, the process skips over the foreach and the DefaultValue process plugin is never called.
5. This causes the default value of any fields that were not set on the D7 site to remain null/empty on the D8 site.

Here is the relevant code from the processRow method in MigrateExecutable.php. You should be able to follow the steps above and see what is happening.

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']) {
    $new_value = [];
    if (!is_array($value)) {
      throw new MigrateException(sprintf('Pipeline failed at %s plugin for destination %s: %s received instead of an array,', $plugin->getPluginId(), $destination, $value));
    }
    $break = FALSE;
    foreach ($value as $scalar_value) {
      try {
        $new_value[] = $plugin->transform($scalar_value, $this, $row, $destination);
      }
      catch (MigrateSkipProcessException $e) {
        $new_value[] = NULL;
        $break = TRUE;
      }
    }
    $value = $new_value;
    if ($break) {
      break;
    }
  }
  else {
    try {
      $value = $plugin->transform($value, $this, $row, $destination);
    }
    catch (MigrateSkipProcessException $e) {
      $value = NULL;
      break;
    }
    $multiple = $plugin->multiple();
  }
}

Here is the relevant code for the Get process plugin. I think pretty much any field will be returned as an array and will therefore set the $multiple variable to TRUE.

public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $source = $this->configuration['source'];
    $properties = is_string($source) ? [$source] : $source;
    $return = [];
    foreach ($properties as $property) {
      if ($property || (string) $property === '0') {
        $is_source = TRUE;
        if ($is_source) {
          $return[] = $row->getSourceProperty($property);
        }
      }
      else {
        $return[] = $value;
      }
    }

    if (is_string($source)) {
      $this->multiple = is_array($return[0]); //This is where multiple is set to TRUE
      return $return[0];
    }
    return $return;
  }

The Solution

Since I don't completely understand how the Migration process is working, I don't know the best way to fix this without potentially negatively impacting something else.

There may be an alternative solution with a different process plugin. I am also not sure if this is intended behavior but if it is, then I believe this behavior should be documented more clearly.