merging an empty vector of meshes / creating an empty mesh

This issue https://github.com/JuliaPlots/Makie.jl/issues/859 happens when an empty vector of meshes is merged. Currently, this errors, but I think it should create an empty Mesh. I don't know if that's currently possible, but I guess it should be, as these edge cases with zero objects happen all the time in dynamic plotting. It should be possible to draw an empty mesh