conman: do not call ssid_get_authentication_mode with empty SSID.

Change-Id: I383b2927c34bbfe2321f2d152b9fc76c2ccae414
diff --git a/conman/interface.py b/conman/interface.py
index e172a82..707890b 100755
--- a/conman/interface.py
+++ b/conman/interface.py
@@ -528,7 +528,8 @@
       client_mode = self._qcsapi('get_mode', 'wifi0') == 'Station'
       ssid = self._qcsapi('get_ssid', 'wifi0')
       status = self._qcsapi('get_status', 'wifi0')
-      security = self._qcsapi('ssid_get_authentication_mode', 'wifi0', ssid)
+      security = (self._qcsapi('ssid_get_authentication_mode', 'wifi0', ssid)
+                  if ssid else None)
     except subprocess.CalledProcessError:
       # If QCSAPI failed, skip update.
       return