fix wait_interactive when there's nothing to wait for

Author: minrk

ipyparallel/client/client.py

@@ -1325,6 +1325,8 @@ def wait_interactive(self, jobs=None, interval=1.0, timeout=-1.0):
         if jobs is None:
             # get futures for results
             futures = [f for f in self._futures.values() if hasattr(f, 'output')]
+            if not futures:
+                return
             ar = AsyncResult(self, futures, owner=False)
         else:
             ar = self._asyncresult_from_jobs(jobs, owner=False)