Ignore data validation if other cell's data has not yet been entered

Hello! I am creating an excel spreadsheet and have a section where the user enters a start date (E1) and another cell where they enter an end date (F1). I have set it using data validation that the date in E1 must be less than the date in F1 and the date in F1 must be greater than the date in E1.

However, I don't want it to do such comparisons when E1 or F1 is blank. Currently, if I delete E1 and then try to enter a new value for F1, it sometimes won't let me depending on the date I enter into F1. If one cell (either E1 or F1) is blank, I want to be able to enter any value in the other cell. Is this possible? How?

Thanks!