Don't iterate options.bot if it isn't present.

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@29220 0039d316-1c4b-4281-b951-d872f2087c98

git-try

@@ -246,8 +246,9 @@ if __name__ == '__main__':
         'svn://svn.chromium.org/chrome-try/try',
     ])
 
-  for bot in options.bot:
-    args.extend(['--bot', bot])
+  if options.bot:
+    for bot in options.bot:
+      args.extend(['--bot', bot])
   if options.clobber:
     args.append('--clobber')
   if options.revision: