Fix redirection to null on windows (#9130)

Concerning generated `bin/release.bat` from `mix release`.

Author: cw789

lib/mix/lib/mix/tasks/release.init.ex

@@ -270,11 +270,11 @@ defmodule Mix.Tasks.Release.Init do
     )
 
     if not "!REL_GOTO!" == "" (
-      findstr "RUNTIME_CONFIG=true" "!RELEASE_SYS_CONFIG!.config" >nil 2>&1 && (
+      findstr "RUNTIME_CONFIG=true" "!RELEASE_SYS_CONFIG!.config" >nul 2>&1 && (
         for /f "skip=1" %%X in ('wmic os get localdatetime') do if not defined TIMESTAMP set TIMESTAMP=%%X
         set RELEASE_SYS_CONFIG=!RELEASE_TMP!\!RELEASE_NAME!-!RELEASE_VSN!-!TIMESTAMP:~0,11!-!RANDOM!.runtime
-        mkdir "!RELEASE_TMP!" >nil
-        copy /y "!REL_VSN_DIR!\sys.config" "!RELEASE_SYS_CONFIG!.config" >nil || (
+        mkdir "!RELEASE_TMP!" >nul
+        copy /y "!REL_VSN_DIR!\sys.config" "!RELEASE_SYS_CONFIG!.config" >nul || (
           echo Cannot start release because it could not write to "!RELEASE_SYS_CONFIG!.config"
           goto end
         )