Fix RunTests to not stall on empty test list

Some PRESUBMIT files dynamically generate test list
and so may pass an empty list to RunTests.  There
appears to be a bug in multiprocessing.Pool that
causes a stall in map_async with an empty list.

TBR=maruel

Review URL: https://chromiumcodereview.appspot.com/14962011

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@200613 0039d316-1c4b-4281-b951-d872f2087c98

presubmit_support.py

@@ -478,7 +478,7 @@ class InputApi(object):
       else:
         assert issubclass(t.message, _PresubmitResult)
         tests.append(t)
-    if parallel:
+    if tests and parallel:
       pool = multiprocessing.Pool()
       # async recipe works around multiprocessing bug handling Ctrl-C
       msgs.extend(pool.map_async(CallCommand, tests).get(99999))