Are you talking about what actually gets saved in the main table?
Checkbox and multiselect joins automatically create a many-to-many mapping tables, named <parent-table>_repeat_<joined-table> ... so for instance I have a checkbox join to us_states on my fab_cdd_test table, so it creates a fab_cdd_test_repeat_us_states table, which has a standard many-to-many map - a parent_id FK pointing back to the parent row, and a us_states FK pointing to the joined us_states row. And for each checkbox selection, a row is created in that map.
But yes, the main table's field (in my case 'us_states') will be null.
That's just a database thing. For a dropdown join, with only a single selection, we can do a simple one-to-one FK join, where the field on the parent table has the FK value pointing to the joined table. But for multiple selections, a many-to-many map table is required. WIthout a map table, you can't have multiple values for a join.
Checkbox elements don't have to join to other tables, so for those we just store the multiple selections as a JSON array.
-- hugh