Make Chat show spinner until ready

Like EditAccount, instead of crashing if the room isn't loaded yet.

src/qml/Models/Rooms.qml

@@ -9,4 +9,24 @@ HListModel {
     sorters: StringSorter {
         roleName: "displayName"
     }
+
+    readonly property ListModel _emptyModel: ListModel {}
+
+    function find(userId, category, roomId) {
+        if (! userId) { return }
+
+        let found = rooms.getWhere({userId, roomId, category}, 1)[0]
+        if (found) { return found }
+
+        return {
+            userId, category, roomId,
+            displayName: "",
+            avatarUrl:   "",
+            topic:       "",
+            members:     _emptyModel,
+            typingText:  "",
+            inviterId:   "",
+            loading:     true,
+        }
+    }
 }

src/qml/Models/Users.qml

@@ -11,8 +11,8 @@ HListModel {
         // the expression with invalid data to establish property bindings
         if (! userId) { return }
 
-        let found = getWhere({userId}, 1)
-        if (found.length > 0) { return found[0] }
+        let found = getWhere({userId}, 1)[0]
+        if (found) { return found }
 
         py.callCoro("request_user_update_event", [userId])