Plotting empty Shape gives DivideError

[goretkin]

This happens under the Plotly backend too. Maybe the behavior could be a no-op, or maybe add an empty series, I'm not sure.

julia> plot(Shape([],[]))
Error showing value of type Plots.Plot{Plots.GRBackend}:
ERROR: DivideError: integer division error
Stacktrace:
 [1] fld at ./int.jl:191 [inlined]
 [2] mod at ./int.jl:171 [inlined]
 [3] mod1 at ./operators.jl:649 [inlined]
 [4] _cycle(::Array{Float64,1}, ::Int64)