Fix goma_auth.bat to use nul instead of null.

Change-Id: I30284634b2c5bdf1d155fe2f8d6f72538c9e2f09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2161634
Auto-Submit: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>

goma_auth.bat

@@ -3,6 +3,6 @@
 :: Use of this source code is governed by a BSD-style license that can be
 :: found in the LICENSE file.
 
-@call "%~dp0\cipd_bin_setup.bat" > null 2>&1
+@call "%~dp0\cipd_bin_setup.bat" > nul 2>&1
 @call python.bat %~dp0\.cipd_bin\goma_auth.py %*
 exit /b