partitions(0) does not return empty set

Currently
```
julia> partitions(0)|>collect
1-element Array{Any,1}:
#undef
```

This is unexpected because the [partition of the empty set is the empty set](https://en.wikipedia.org/wiki/Partition_of_a_set#Examples) so I would expect
```
patitions(0)|>collect == [[]]
```

This results in unexpected behviour here:
```
julia> [length(b) for b in partitions(1)]
1-element Array{Int64,1}:
1

julia> [length(b) for b in partitions(0)]
1-element Array{Int64,1}:
4492284592

# I expect this
julia> [length(b) for b in [[]]]
1-element Array{Int64,1}:
0
```