column_null.twig

<input name="field_null[{{ column_number }}]"
    id="field_{{ column_number }}_{{ ci - ci_offset }}"
    {% if column_meta['Null'] is not empty and column_meta['Null'] != 'NO' and column_meta['Null'] != 'NOT NULL' -%}
        checked="checked"
    {%- endif %}
    type="checkbox"
    value="NULL"
    class="allow_null" />