Correct. In normal Julia you need to pass the init keyword:
julia> a = [1, 2, 3, 4]
4-element Vector{Int64}:
1
2
3
4
julia> sum(a[n] for n in 1:0; init = 0)
0
We have this set by default in JuMP because we know that people really want 0.0 when they have an empty summation.