Why would you want to add a product with quantity zero? I don't see how this can help you tracking something as Odoo will not make a stock move line for this SO line. Is there maybe a better solution? If you share more details of the use case, we can think along with you.

If you really want to add a SO line with quantity 0 you could think about editing the sales order report with Odoo Studio. You can hide a line on the report when the quantity is 0.

You could modify the QWeb report to only display the sales order line when the quantity is not zero. You would do this in the view report_saleorder_document. Somewhere around here

<tr t-att-class="'bg-200 font-weight-bold o_line_section' if line.display_type == 'line_section' else 'font-italic o_line_note' if line.display_type == 'line_note' else ''">
                            <t t-if="not line.display_type">
                                <td name="td_name"><span t-field="line.name"/></td>