\Drupal\migrate\ProcessPluginBase::stopPipeline calls should return NULL

Problem/Motivation

The change record Migrate process plugin can now stop the process pipeline after they run examples show returning NULL after \Drupal\migrate\ProcessPluginBase::stopPipeline is called, but the implementations in do not return NULL after the stop call. This leads to the values passed to the file_blob, gate, and skip_on_value plugins being returned by those plugins when the pipeline is stopped, which MigrateExecutable then saves.

Steps to reproduce

Create a migration with the following pipeline:

field_taxonomy:
  - plugin: skip_on_value
    source: taxonomy
    value:
      - 'Skip One'
      - 'Skip Two'
  - plugin: entity_generate
    entity_type: taxonomy_term
    value_key: name
    bundle_key: vid
    bundle: vocabulary
    ignore_case: true

Note that even if the term matches the values to skip, the value of the taxonomy source field will attempt to be inserted into field_taxonomy causing a database exception.

Proposed resolution

Add return NULL; after calls to \Drupal\migrate\ProcessPluginBase::stopPipeline.