Fix no availability zone error

When there's no availability zone on creating server, we should
pass None instead of "" to make it work.

Change-Id: I5adbc82f120bf631c9bbf264613a0b2d0dfbaac7

mogan_ui/content/servers/workflows/create_server.py

@@ -365,6 +365,8 @@ class LaunchServer(workflows.Workflow):
             nics = None
 
         avail_zone = context.get('availability_zone', None)
+        if not avail_zone:
+            avail_zone = None
 
         try:
             mogan.server_create(request,