do not try collecting issues information for an empty issues list

There is no point in trying to retrieve issues information when the
issues list is empty, On top of that the attempt to start a thread
pool with zero threads generates an exception.

TEST=running my_activity.py when the user does not have issues on
     certain servers does not cause the exception any more.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Change-Id: Ib5889a4c3390a01860fb31a4ff1a343e05b24051
Reviewed-on: https://chromium-review.googlesource.com/1058679
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>

my_activity.py

@@ -695,6 +695,9 @@ class MyActivity(object):
           self.monorail_issue_search, monorail_projects.keys())
       monorail_issues = list(itertools.chain.from_iterable(monorail_issues))
 
+    if not monorail_issues:
+      return
+
     with contextlib.closing(ThreadPool(len(monorail_issues))) as pool:
       filtered_issues = pool.map(
           self.filter_modified_monorail_issue, monorail_issues)