Changed in version 12.0: This option was introduced to replace the TCA
eval option with
null as value.
If set to true, a checkbox will appear, which by default deactivates the field. In the deactivated state the field is saved as
NULLin the database. By activating the checkbox it is possible to set a value, which won't be saved as
NULL, even an empty string.
The database field should have the according
'columns' => [ 'nullable_column' => [ 'title' => 'A nullable field', 'config' => [ 'type' => 'link', 'nullable' => true, ], ], ],