prosodyclient: Use empty name if none provided by the server

In parallel, I have updated the server to use the group name for groups with
no name (the MUCs with no name are typically the default auto-created group
MUC).

snikket_web/prosodyclient.py

@@ -190,7 +190,7 @@ class AdminGroupChatInfo:
         return cls(
             id_=data["id"],
             jid=data["jid"],
-            name=data["name"],
+            name=data.get("name", ""),
         )