Do not provide default value for profiling

Author: josevalim

lib/mix/lib/mix/cli.ex

@@ -10,7 +10,7 @@ defmodule Mix.CLI do
     if env_variable_activated?("MIX_QUIET"), do: Mix.shell(Mix.Shell.Quiet)
     if env_variable_activated?("MIX_DEBUG"), do: Mix.debug(true)
 
-    if profile = System.get_env("MIX_PROFILE", "") do
+    if profile = System.get_env("MIX_PROFILE") do
       Mix.State.put(:profile, String.split(profile, ","))
     end