Do not add new line on all empty mix format

Author: josevalim

lib/mix/lib/mix/tasks/format.ex

@@ -544,10 +544,11 @@ defmodule Mix.Tasks.Format do
   defp stdin_or_wildcard("-"), do: [:stdin]
   defp stdin_or_wildcard(path), do: path |> Path.expand() |> Path.wildcard(match_dot: true)
 
-  defp elixir_format("", _formatter_opts), do: ""
-
   defp elixir_format(content, formatter_opts) do
-    IO.iodata_to_binary([Code.format_string!(content, formatter_opts), ?\n])
+    case Code.format_string!(content, formatter_opts) do
+      [] -> ""
+      _ -> IO.iodata_to_binary([Code.format_string!(content, formatter_opts), ?\n])
+    end
   end
 
   defp find_sigils_from_plugins(plugin, formatter_opts) do

lib/mix/test/mix/tasks/format_test.exs

@@ -15,13 +15,15 @@ defmodule Mix.Tasks.FormatTest do
     end
   end
 
-  test "doesn't format empty lines into line breaks", context do
+  test "doesn't format empty files into line breaks", context do
     in_tmp(context.test, fn ->
       File.write!("a.exs", "")
-
       Mix.Tasks.Format.run(["a.exs"])
-
       assert File.read!("a.exs") == ""
+
+      File.write!("b.exs", "  \n  \n")
+      Mix.Tasks.Format.run(["b.exs"])
+      assert File.read!("b.exs") == ""
     end)
   end