If the arguments are not as expected (e.g. arrays of different lengths, both scalars, not numeric) then null is returned.
arrayOrScalar1 + arrayOrScalar2
,
the same length as the input array(s)add(array(1,2,3), array(0.1,0.2,0.3))
= [1.1, 2.2, 3.3]
add(array(1,2,3), 10) = [11,12,13]