Want cell to return no result

Use an IF perhaps ?

IF(AB1="","",SUM(X1,AE1))

On an aside... the advantage to using SUM over + is that it does not explicitly coerce... for ex.

X1 = "apple" (user error)
AE1 is 10

=X1+AE1 --> #VALUE! as X1 is not numeric...
=SUM(X1,AE1) --> 10 given the SUM will only look at numeric values...

my point... there is thus little value in using

=SUM(X1+AE1)

as the benefits of the SUM are negated by the explicit coercion taking place within the SUM itself therefore if using X1+AE1 dispense with SUM and conversely if using the SUM dispense with the + operator and use , to delimit the ranges to be summed
(you are constrained in terms of how many areas you may delimit in the SUM)