django22: django.test.client.encode_multipart no longer accepts None

As seen here [1].

When a corresponding field has no selection, an empty string is
passed to the server. This change replaces None with an empty string.

After this change, all tests pass with Django 2.2,
so we can make django22 job voting.

[1] https://github.com/django/django/commit/6fe9c45b725

Change-Id: I4cd551253ac361eaa46b6de35369125eb0e20e98

.zuul.yaml

@@ -10,8 +10,8 @@
     check:
       jobs:
         - horizon-openstack-tox-python3-django111
-        - horizon-openstack-tox-python3-django22:
-            voting: false
+        - horizon-openstack-tox-python3-django22
     gate:
       jobs:
         - horizon-openstack-tox-python3-django111
+        - horizon-openstack-tox-python3-django22

neutron_fwaas_dashboard/dashboards/project/firewalls_v2/tests.py

@@ -331,7 +331,7 @@ class FirewallTests(test.TestCase):
         rules = self.fw_rules_v2.list()
         tenant_id = self.tenant.id
         form_data = {'description': policy.description,
-                     'firewall_rules': None,
+                     'firewall_rules': '',
                      'shared': policy.shared,
                      'audited': policy.audited
                      }