`getindex` fails with empty `Union{}` array

[simonbyrne]

julia> X = [1,2,3];

julia> X[Union{}[]]
ERROR: ArgumentError: invalid index: Union{}[] of type Array{Union{},1}
Stacktrace:
 [1] to_index(::Array{Union{},1}) at ./indices.jl:262
 [2] to_index(::Array{Int64,1}, ::Array{Union{},1}) at ./indices.jl:247
 [3] to_indices at ./indices.jl:293 [inlined]
 [4] to_indices at ./indices.jl:290 [inlined]
 [5] getindex(::Array{Int64,1}, ::Array{Union{},1}) at ./abstractarray.jl:905
 [6] top-level scope at none:0

cause of JuliaData/JuliaDB.jl#228