Only set column type for "additional_data" if column exists

[Staubiii]

We didnt add this field (yet) and ran into a problem while upgrading to CakePHP 5.x and with it to this version of the users plugin.

Column additional_data of table users: The column type json can only be set if the column already exists; can be checked using hasColumn().

So we propose to only set the column type when the column actually exists.