Skip monitoring ports when providing empty list of port

Change-Id: Id40690eb7eaf755f49b8996206513eb0bdf96c18
diff --git a/TestON/drivers/common/api/controller/trexclientdriver.py b/TestON/drivers/common/api/controller/trexclientdriver.py
index 50c8c39..1150d4c 100644
--- a/TestON/drivers/common/api/controller/trexclientdriver.py
+++ b/TestON/drivers/common/api/controller/trexclientdriver.py
@@ -364,15 +364,18 @@
 
         :param ports: List of ports ids to monitor
         :param time_interval: Interval between read
-        :return: Statistics read while traffic is active
+        :return: Statistics read while traffic is active, or empty result if no
+                 ports provided.
         """
-
         results = {
             port_id: {"rx_bps": [], "tx_bps": [], "rx_pps": [], "tx_pps": []}
             for port_id in ports
         }
         results["duration"] = []
 
+        if not ports:
+            return results
+
         prev = {
             port_id: {
                 "opackets": 0,