Do not write any config for a wlan device that has not been given a ESSID

See https://bugs.launchpad.net/ubuntu/+source/subiquity/+bug/1623756

subiquitycore/models/network.py

@@ -119,7 +119,8 @@ class Networkdev():
             result[self.ifname]['interfaces'] = self.probe_info.bond['slaves']
 
         if self.iftype == 'wlan':
-            if self.essid is not None:
+            if self.essid is None:
+                return {}
             aps = result[self.ifname]['access-points'] = {}
             ap = aps[self.essid] = {
                 'mode': 'infrastructure',