\Drupal\migrate\Plugin\migrate\process\StaticMap::transform() cannot map NULL values unless there is a default value or bypass is set

Currently the static map plugin will map NULL to values using an empty string array key. For example the following StaticMap configuration

map:
  '': 'Mapped null value'
  this: that

will map NULL values to 'Mapped null value'. This will trigger a deprecation in Drupal 11.3.0. To avoid the deprecation the migration can either use the default_value or bypass setting. Setting bypass to TRUE will return NULLs for NULL values. Setting a default_value will return the default value for NULL values.