can't `cat` an empty vector

[bjarthur]

cat works fine if both vectors are not-empty:

julia> a=rand(X([1,2,3]))
3-element DimArray{Float64,1} with dimensions: 
  X Sampled{Int64} Int64[1, 2, 3] ForwardOrdered Irregular Points
 1  0.3884
 2  0.454392
 3  0.768688

julia> b=rand(X([4,5]))
2-element DimArray{Float64,1} with dimensions: 
  X Sampled{Int64} Int64[4, 5] ForwardOrdered Irregular Points
 4  0.828142
 5  0.50528

julia> cat(a,b,dims=X)
5-element DimArray{Float64,1} with dimensions: 
  X Sampled{Int64} Int64[1, 2, 3, 4, 5] ForwardOrdered Irregular Points
 1  0.3884
 2  0.454392
 3  0.768688
 4  0.828142
 5  0.50528

but throws an error if one is empty:

julia> c=rand(X(Int64[]))
0-element DimArray{Float64,1} with dimensions: 
  X Sampled{Int64} Int64[] Unordered Irregular Points

julia> cat(c,b,dims=X)
ERROR: BoundsError: attempt to access 0-element Vector{Int64} at index [0]
Stacktrace:
  [1] getindex
    @ Base ./essentials.jl:13 [inlined]
  [2] last
    @ Base ./abstractarray.jl:524 [inlined]
  [3] last
    @ DimensionalData.Dimensions.LookupArrays ~/.julia/packages/DimensionalData/R7veM/src/LookupArrays/lookup_arrays.jl:30 [inlined]
  [4] _check_cat_lookup_order
    @ DimensionalData ~/.julia/packages/DimensionalData/R7veM/src/array/methods.jl:391 [inlined]
  [5] _check_cat_lookups(D::Type, l1::DimensionalData.Dimensions.LookupArrays.Sampled{…}, lookups::DimensionalData.Dimensions.LookupArrays.Sampled{…})
    @ DimensionalData ~/.julia/packages/DimensionalData/R7veM/src/array/methods.jl:353
  [6] check_cat_lookups(::X{…}, ::Vararg{…})
    @ DimensionalData ~/.julia/packages/DimensionalData/R7veM/src/array/methods.jl:346
  [7] (::DimensionalData.var"#127#134"{Tuple{…}, DimArray{…}, Tuple{…}})(catdim::Type)
    @ DimensionalData ~/.julia/packages/DimensionalData/R7veM/src/array/methods.jl:251
  [8] map
    @ ./tuple.jl:291 [inlined]
  [9] _cat(catdims::Tuple{…}, A1::DimArray{…}, As::DimArray{…})
    @ DimensionalData ~/.julia/packages/DimensionalData/R7veM/src/array/methods.jl:227
 [10] _cat(catdim::Type{…}, A1::DimArray{…}, As::DimArray{…})
    @ DimensionalData ~/.julia/packages/DimensionalData/R7veM/src/array/methods.jl:223
 [11] cat(A1::DimArray{…}, As::DimArray{…}; dims::Type)
    @ DimensionalData ~/.julia/packages/DimensionalData/R7veM/src/array/methods.jl:220
 [12] top-level scope
    @ REPL[76]:1
Some type information was truncated. Use `show(err)` to see complete types.

works fine for regular (ie non-DimArray) arrays:

julia> cat([],[3,4],dims=1)
2-element Vector{Any}:
 3
 4