skip_on_empty doesn't work if the value is an empty array

Problem/Motivation

Using the content_entity migration source, which takes entities from the local Drupal site, this configuration for a process mapping will never skip an empty field value:

  dummy_skip:
    plugin: skip_on_empty
    method: row
    source: my_field

This is because the field value when the field has no data is an empty array. The implicit 'get' plugin loads this, and because it's an array, MigrateExecutable::processRow() sets $multiple to TRUE.

That means that when processRow() comes to the skip_on_empty plugin, it iterates over the array, to pass each value to the plugin.

There are no values in the array, hence the skip_on_empty plugin is never called.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet