ms_srs: fix additional_args init

regressions after porting to list. init to new empty list

Change-Id: I06f370462934ca35f3aa13bbfcd9e5fc3df91895
diff --git a/src/osmo_gsm_tester/obj/ms_srs.py b/src/osmo_gsm_tester/obj/ms_srs.py
index 6241685..cf2e166 100644
--- a/src/osmo_gsm_tester/obj/ms_srs.py
+++ b/src/osmo_gsm_tester/obj/ms_srs.py
@@ -230,11 +230,10 @@
         self.enable_pcap = util.str2bool(values['ue'].get('enable_pcap', 'false'))
         config.overlay(values, dict(ue={'enable_pcap': self.enable_pcap}))
 
-        self._additional_args = values['ue'].get('additional_args', '').split()
+        self._additional_args = []
         for add_args in values['ue'].get('additional_args', []):
             self._additional_args += add_args.split()
 
-
         self.num_carriers = int(values['ue'].get('num_carriers', 1))
 
         # We need to set some specific variables programatically here to match IP addresses: