Instead of checking for null, I checked if the date was greater than a date far in the past. I wrapped the basic condition generated automatically by flow inside the coalesce function (you have to switch to advanced mode). The coalesce function will not return a null. If your date field is null it returns a "fallback" value that you set in the function. Just set the fallback date less than the date in your condition. In the example below I check to see if my ReceiveDate field is greater than 1/1/1991. If yes then I send an email: