[moblab] Fix bug in moblab run suite RPC.

If the RPC is called with no suite_args the code path is incorrect
and results in no tests running / error message being recieved.

BUG=chromium:728846
TEST=Unit test passes, local testing on moblab.

Change-Id: Ibc83a38c0cb232ea15c126fdafc4e2fb3d2ffb0c
Reviewed-on: https://chromium-review.googlesource.com/521754
Commit-Ready: Keith Haddow <haddowk@chromium.org>
Tested-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Jinsong Mu <jinsong@google.com>
Reviewed-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Michael Tang <ntang@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
(cherry picked from commit c4273f917c12e19751dc0846d25f96f4a54c7b49)
Reviewed-on: https://chromium-review.googlesource.com/527275
diff --git a/frontend/afe/moblab_rpc_interface.py b/frontend/afe/moblab_rpc_interface.py
index d4cfda1..91645b5 100644
--- a/frontend/afe/moblab_rpc_interface.py
+++ b/frontend/afe/moblab_rpc_interface.py
@@ -754,7 +754,10 @@
         builds['fwrw-version'] = rw_firmware
     if ro_firmware:
         builds['fwro-version'] = ro_firmware
-    list_suite_args = map(lambda s: s.strip(), suite_args.split(','))
+    if suite_args:
+        list_suite_args = map(lambda s: s.strip(), suite_args.split(','))
+    else:
+        list_suite_args = None
     afe = frontend.AFE(user='moblab')
     afe.run('create_suite_job', board=board, builds=builds, name=suite,
     pool=pool, run_prod_code=False, test_source_build=build,