Support TriangularToeplitz(Float64[],:U)

[dlfivefifty]

TriangularToeplitz(Float64[],:U) currently gives an unreadable error. I think it should be supported (to represent a 0 x 0 matrix), or otherwise given a better error.

Supporting it will take some thought about how to construct tmp.

[andreasnoack]

I think the main challenge is the dft since we could just condition https://github.com/JuliaMatrices/ToeplitzMatrices.jl/blob/master/src/ToeplitzMatrices.jl#L366-L373 on n>0. We might want to avoid the cost of creating the dft for small n>0 so maybe a way forward is to make that field Nullable{Base.DFT.FFTW.cFFTWPlan{Complex{T},-1,true,1}} or something like that.

[dlfivefifty]

Yes I think that's a good idea. (I forgot about Nullable...)

[dlfivefifty]

For my usage, I think I can work around this by using Nullable{TriangularToeplitz}.

[dkarrasch]

The following works, so I believe the issue has been solved some time ago.

julia> U = TriangularToeplitz(Float64[],:U)
0×0 TriangularToeplitz{Float64}

julia> x = Float64[]
Float64[]

julia> U*x
Float64[]