Should broadcast extrude to match length 0 dimensions

[mbauman]

Brought up by the ever vigilant @bkamins, we currently support broadcast in some seemingly surprising cases:

julia> rand(5, 0, 5) .+ (1:5)
5×0×5 Array{Float64,3}

julia> rand(5, 0, 5) .+ 1
5×0×5 Array{Float64,3}

julia> rand(5, 0, 5) .+ (1:5)
5×0×5 Array{Float64,3}

julia> rand(5, 0, 5) .+ ones(5,1,1)
5×0×5 Array{Float64,3}

Perhaps even more surprising is this:

julia> x = []
0-element Array{Any,1}

julia> x .= 1
0-element Array{Any,1}

julia> [] .= [1]
0-element Array{Any,1}

I don't think this is horrible enough to warrant a breaking change before 2.0, but perhaps may be something to consider changing in the future.

[bkamins]

As a comment to explain how I understand what we do now.
We allow dimensions that have length 1 to be expanded. In particular it means that we allow them to be expanded to 0 (as expansion is multiplication by the length of the dimension we want to expand to).

Here is a sample extreme case:

julia> rand(1,0,0) .+ rand(0,1,0) .+ rand(0,0,1)
0×0×0 Array{Float64,3}