fix plotting empty plot for plotly

src/backends/plotly.jl

@@ -1031,6 +1031,9 @@ end
 
 # get a list of dictionaries, each representing the series params
 function plotly_series(plt::Plot)
+    if isempty(plt.series_list)
+        return KW[]
+    end
     reduce(vcat, plotly_series(plt, series) for series in plt.series_list)
 end
 

test/test_plotly.jl

@@ -7,6 +7,7 @@ using Plots, Test
 
         p = plot(rand(10))
         @test isa(p, Plots.Plot) == true
+        @test_nowarn Plots.plotly_series(plot())
     end
 
     @testset "Contours" begin