I have a spreadsheet set up for GPA but how do I enter a null value? I have
the formula set up as (which shows in F26)

=LOOKUP(D26,{"a","a-","b+","b","b-","c+","c","c-","d+","d","d-","f";4,3.7,3.3,3,2.7,2.3,2,1.7,1.3,1,.7,0})*E 26

now if the value in D27 is empty how do I show a null value in F27