Excel MAX Null Value

I have a formula working pretty well that will lookup a value into a range of data and then retrieve the max value from a different range (it's student test scores that they submit through a GoogleForm, I'm searching for their email addresses and only want to pull their top score through to a dashboard (they can sit the test multiple times)).

This all works well but I'm stuck on the last bit. Instead of the max value of students who haven't taken the test yet (and therefore not having an email address in the lookup range) appearing as 0, I'd like it to be blank (or anything else). This is because I need to be able to see which students have taken the test, have an email address in the responses sheet to be looked up and scored 0 (!) and those who haven't taken the test at all - at the moment there is no differentiation.

I've tried all different combos of IF like I normally would but can't crack this!

Here's the formula I have that works perfectly apart from the last bit!