Repair `cell` in matrixbox TALES context

This repairs TALES expressions using constructs like `cell is not None` to test if a cell exists, like for example:

https://lab.nexedi.com/nexedi/erp5/blob/1b706b6c29771b5fcbde6499be5df905ca503d5a/bt5/erp5_open_trade/SkinTemplateItem/portal_skins/erp5_open_trade/OpenOrderLine_viewPrice/matrixbox_price.xml#L110


https://lab.nexedi.com/nexedi/erp5/blob/1b706b6c29771b5fcbde6499be5df905ca503d5a/bt5/erp5_pdm/SkinTemplateItem/portal_skins/erp5_pdm/Measure_view/matrixbox_quantity.xml#L110



See merge request nexedi/erp5!1448