fix wtf crash when deps entry is at None

There are several places where we advise people
to use custom_deps with { "foo" : None } to avoid
checking out certain sub-repos.  This caused wtf to
crash on startup.


BUG=


Review URL: https://chromiumcodereview.appspot.com/10917220

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@156373 0039d316-1c4b-4281-b951-d872f2087c98

wtf

@@ -64,7 +64,7 @@ def main():
   root, entries = gclient_utils.GetGClientRootAndEntries()
 
   # which entries map to a git repos?
-  raw = [k for k, v in entries.items() if not re.search('svn', v)]
+  raw = [k for k, v in entries.items() if v and not re.search('svn', v)]
   raw.sort()
 
   # We want to use the full path for testing, but we want to use the relative