Fix: Optional MorphToSelect does not set key column to null

[coleshirley]

Description

I have an optional MorphToSelect field:

MorphToSelect::make('morphable')
    ->types([
        MorphToSelect\Type::make(MorphType1::class)
            ->titleAttribute('name'),
        MorphToSelect\Type::make(MorphType2::class)
            ->titleAttribute('name'),
        MorphToSelect\Type::make(MorphType3::class)
            ->titleAttribute('name'),
    ]),

When I set the type field back to "Select an option" and save the morphable_id column is still set even though the morphable_type is set to null.

This PR adds an override of the dehydrateState() method that manually sets the "key" column to null if the field is not required

Code style

• composer cs command has been run.

Testing

• Changes have been tested.

Documentation

• Documentation is up-to-date.

[danharrin]

Thanks! I've given this a refactor and a few tests to make it an official API, instead of overriding just for this component