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
1 file changed