Allow just measuring memory

Made best_unit more robust as that's where it was blowing up

lib/benchee/conversion/scale.ex

@@ -137,12 +137,32 @@ defmodule Benchee.Conversion.Scale do
       iex> list = [1, 101, 1_001, 10_001, 100_001, 1_000_001]
       iex> Benchee.Conversion.Scale.best_unit(list, Benchee.Conversion.Count, strategy: :largest).name
       :million
+
+      iex> list = []
+      iex> Benchee.Conversion.Scale.best_unit(list, Benchee.Conversion.Count, strategy: :best).name
+      :one
+
+      iex> list = [nil]
+      iex> Benchee.Conversion.Scale.best_unit(list, Benchee.Conversion.Count, strategy: :best).name
+      :one
+
+      iex> list = [nil, nil, nil, nil]
+      iex> Benchee.Conversion.Scale.best_unit(list, Benchee.Conversion.Count, strategy: :best).name
+      :one
+
+      iex> list = [nil, nil, nil, nil, 2_000]
+      iex> Benchee.Conversion.Scale.best_unit(list, Benchee.Conversion.Count, strategy: :best).name
+      :thousand
   """
-  def best_unit([], module, _) do
+  def best_unit(measurements, module, options) do
+    do_best_unit(Enum.reject(measurements, &is_nil/1), module, options)
+  end
+
+  defp do_best_unit([], module, _) do
     module.base_unit
   end
 
-  def best_unit(list, module, opts) do
+  defp do_best_unit(list, module, opts) do
     case Keyword.get(opts, :strategy, :best) do
       :best     -> best_unit(list, module)
       :largest  -> largest_unit(list, module)

test/benchee_test.exs

@@ -418,6 +418,20 @@ defmodule BencheeTest do
     refute output =~ "never execute me"
   end
 
+  test "does not blow up when only measuring memory times" do
+    output = capture_io fn ->
+      Benchee.run(
+        %{
+          "something" => fn -> Enum.map(1..100, fn i -> i + 1 end) end
+        },
+        time: 0, warmup: 0, memory_time: 0.001
+      )
+    end
+
+    refute output =~ ~r/ips/i # no runtime statistics displayed
+    assert output =~ ~r/memory.+statistics/i
+  end
+
   describe "save & load" do
     test "saving the suite to disk and restoring it" do
       save = [save: [path: "save.benchee", tag: "master"]]