Issue #3197749 by quietone, joachim: empty migrate source does not populate fields with constants

 *     field_name: image

 * @endcode

 *

 * This will return a single row containing 'entity_type' and 'field_name'

 * elements, with values of 'user' and 'image', respectively.

 * This will return a single row containing 'constants/entity_type' and

 * 'constants/field_name' elements, with values of 'user' and 'image',

 * respectively.

 *

 * @MigrateSource(

 *   id = "empty",

 *   (optional table alias). This high_water_property is typically a timestamp

 *   or serial id showing what was the last imported record. Only content with a

 *   higher value will be imported.

 * - constants: (optional) An array of constants that can be used in the process

 *   pipeline. To use the constant 'foo' as a source value use 'constants/foo'.

 *

 * The high_water_property and track_changes are mutually exclusive.

 *

 * migration. This will get converted into a SQL condition that looks like

 * 'n.changed' or 'changed' if no alias.

 *

 * Example:

 *

 * @code

 * source:

 *   plugin: some_source_plugin_name

 *   constants:

 *     - foo: bar

 * process:

 *   baz: constants/bar

 * @endcode

 *

 * In this example, the constant 'foo' is defined with a value of 'bar'. It is

 * later used in the process pipeline to set the value of the field baz.

 *

 * @see \Drupal\migrate\Plugin\MigratePluginManager

 * @see \Drupal\migrate\Annotation\MigrateSource

 * @see \Drupal\migrate\Plugin\MigrateSourceInterface