No Format Conditional Formatting Online

Im trying to have a conditional format set to show any dates including today and before. I have that set but its highlighting every blank cell as well. I want to have no format set for the blank cell and stop if true to avoid it being overruled, but looking up how to fix it seems to only work with desktop excel, not online. I cant find a way to set no format online because the default is formatted. I also cant just set it to be white because some of the blank cells are filled for another reason. Anyone know if you can set no format in online excel?