Nullable Behavior

The behavior replaces an empty string with the schema default only when the column allows NULL. A nullable column with a NULL default becomes null; a nullable column with a non-null default receives that default. Non-nullable columns, unknown fields, and non-empty values are unchanged.

// In your Table::initialize().
$this->addBehavior('Shim.Nullable');

For a nullable column with a NULL default, queries can check IS NULL after normalization:

$query->where(['field_name IS' => null]);

The behavior does not rewrite existing records. Clean up historical empty strings separately if you need a consistent database value.

Callback options

You can define the time this should happen.

Default callback affects validation

Be careful with the default setting (beforeMarshal), as this can affect validation rules. Some rules use the empty-string existence to validate fields, through isset() checks. Those checks might not work as expected if the input is already back to null. There you would then need array_key_exists().

Another option is to apply the data-integrity cleanup on beforeSave instead:

// In your Table
$this->addBehavior('Shim.Nullable', ['on' => 'beforeSave']);