Fix case where there is nothing to uprev.

TEST=Ran unit tests.

Change-Id: Iafd9702c5364511d79667ecc296eaf68fc89a5fb

Review URL: http://codereview.chromium.org/3159047

bin/cbuildbot.py

@@ -209,6 +209,9 @@ def _ParseRevisionString(revision_string, repo_dictionary):
 
 def _UprevFromRevisionList(buildroot, revision_list):
   """Uprevs based on revision list."""
+  if not revision_list:
+    print >> sys.stderr, 'No packages found to uprev'
+    return
   package_str = ''
   commit_str = ''
   for package, revision in revision_list:

bin/cbuildbot_unittest.py

@@ -29,6 +29,7 @@ class CBuildBotTest(mox.MoxTestBase):
                        'cros' : ['chromos-base/libcros']
                       }
     self._test_string = "kernel.git@12345test cros.git@12333test"
+    self._test_string += " crosutils.git@blahblah"
     self._revision_file = 'test-revisions.pfq'
     self._test_parsed_string_array = [
                                       ['chromeos-base/kernel', '12345test'],