Add empty ViewAxis

This should be possible:
```julia
julia> x = ComponentArray(a=[], b=3.5, c=[15, 2]);

julia> x.a
0-element Array{Float64,1}
```
Should be doable by just writing a method for indexing into a `ComponentArray` with `ComponentIndex{Nothing,AbstractAxis}` and a method of `make_carray_args` that creates a `ViewAxis` with `nothing` as the view index. 