Empty products_image will use the no image ... and you can turn off the required ...
However, to show the no image when an image is missing but defined you would need to alter the zen_image function slightly around this section of code:
PHP Code:
//auto replace with defined missing image
if ($src == DIR_WS_IMAGES and PRODUCTS_IMAGE_NO_IMAGE_STATUS == '1') {
$src = DIR_WS_IMAGES . PRODUCTS_IMAGE_NO_IMAGE;
}
by setting a condition that when the image is missing but defined it also shows the no image ...
Be careful as the template also uses this function and you don't want to confuse the evaluation of the testing ...