Bug in integer_partitions(0)

`integer_partitions(0)` returns the empty list, while there is a partition of 0, namely `Int[]`.

The error is in src/partitions.c:459, which should return `Vector{Int}[[]]` or `[Int[]]`.

There is also a problem with `partitions(0)`:
```julia
julia> partitions(0)
Combinatorics.IntegerPartitions(0)

julia> collect(ans)
1-element Vector{Vector{Int64}}:
#undef

julia> integer_partitions(0)
Vector{Int64}[]
```