Issue #3336070 by mikelutz, efpapado, smustgrave, quietone, danflanagan8:...

 * - method: (optional) What to do if the input value is empty. Possible values:

 *   - row: Skips the entire row when an empty value is encountered.

 *   - process: Prevents further processing of the input property when the value

 *     is empty.

 *     is empty and sets the value to NULL.

 * - message: (optional) A message to be logged in the {migrate_message_*} table

 *   for this row. Messages are only logged for the 'row' method. If not set,

 *   nothing is logged in the message table.