Add unique constraint to email branding domains

Does two things:

1. Revert "Revert "Add unique constraint to email branding domain""

This reverts commit af9cb30ef3.

2. Don’t allow empty string in email branding domain

Columns with multiple `null`s can have a uniqueness constraint. Columns
with multiple empty string values are not considered unique.

This commit:
- removes any duplicate empty string values
- casts empty strings to null string any time these columns are updated

---

Squashed into this single commits because these two things are not
atomic as individual commits.

tests/app/dao/test_email_branding_dao.py

@@ -51,3 +51,15 @@ def test_update_email_branding(notify_db, notify_db_session):
 
     assert len(email_branding) == 1
     assert email_branding[0].name == updated_name
+
+
+def test_domain_cant_be_empty_string(notify_db, notify_db_session):
+    create_email_branding()
+
+    email_branding = EmailBranding.query.all()
+    assert email_branding[0].domain is None
+
+    dao_update_email_branding(email_branding[0], domain='')
+
+    email_branding = EmailBranding.query.all()
+    assert email_branding[0].domain is None