Fix a None reference exception when searching for presubmit scripts.

It happens when a user uses --diff foo.diff so the list of file is not defined.

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/3763004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@62547 0039d316-1c4b-4281-b951-d872f2087c98

gcl.py

@@ -866,7 +866,7 @@ def TryChange(change_info, args, swallow_exception):
     file_list = change_info.GetFileNames()
   else:
     trychange_args.extend(args)
-    file_list = None
+    file_list = []
   return trychange.TryChange(
       trychange_args,
       file_list=file_list,