conv corner case

[KristofferC]

Moved from https://github.com/JuliaLang/julia/issues/20539

conv( randn(5), randn(0) ), say should be, i'd think, should be a vector of length 0 right now it's an error

conv(randn(0), randn(0)) is trickier, but a vector length 0 is not unreasonable