use === nothing

Should decrease compile times

test/imgcomp.jl

@@ -64,7 +64,7 @@ function image_comparison_tests(pkg::Symbol, idx::Int; debug = false, popup = is
 
     # now we have the fn (if any)... do the comparison
     # @show reffn
-    if reffn == nothing
+    if reffn === nothing
         reffn = newfn
     end
     # @show reffn
@@ -98,7 +98,7 @@ function image_comparison_facts(pkg::Symbol;
                                 tol = 1e-2)     # acceptable error (percent)
   for i in 1:length(Plots._examples)
     i in skip && continue
-    if only == nothing || i in only
+    if only === nothing || i in only
       @test image_comparison_tests(pkg, i, debug=debug, sigma=sigma, tol=tol) |> success == true
     end
   end