Provide a way to unset empty Migrate Row destinations

Not sure if this is 100% the same problem, but I solved this by completely rejecting migration mappings from entering the database if arbitrary criteria was not met. I did this by extending the skip_on_empty plugin to what I call exclude_on_empty.

class ExcludeOnEmpty extends SkipOnEmpty {

  /**
   * Excludes the current row when value is not set.
   *
   * {@inheritDoc}
   */
  public function row($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    try {
      return parent::row($value, $migrate_executable, $row, $destination_property);
    }
    catch (MigrateSkipRowException $e) {
      throw new MigrateSkipRowException($e->getMessage(), FALSE);
    }
  }

}

Here's a process-plugin approach to the same basic problem: ✨ Add set_on_condition process plugin Needs review

I'm not sure it's a good idea, but I wanted to throw it out there.

codebymikey → closed merge request !2213

Thanks @danflanagan8, I think this issue addresses a different situation, which is related to the fact that the empty value gets out of sync with the actual state of the row field.

I've updated the issue summary accordingly.