Set forward_destination to an empty list

The value of `None` resulted in an error, since a list was expected.

core/admin/mailu/models.py

@@ -267,7 +267,7 @@ class User(Base, Email):
 
     # Filters
     forward_enabled = db.Column(db.Boolean(), nullable=False, default=False)
-    forward_destination = db.Column(CommaSeparatedList(), nullable=True, default=None)
+    forward_destination = db.Column(CommaSeparatedList(), nullable=True, default=[])
     forward_keep = db.Column(db.Boolean(), nullable=False, default=True)
     reply_enabled = db.Column(db.Boolean(), nullable=False, default=False)
     reply_subject = db.Column(db.String(255), nullable=True, default=None)