Add hack to fix room sidebar not sticking to the top

This is similar to the TimelineView hacks, but this time its the header
item that's changing height as our room topics and such wrap.

(cherry picked from commit 6eb2b2e739)

src/libneochat/qml/GroupChatDrawerHeader.qml

@@ -22,9 +22,9 @@ ColumnLayout {
     /**
      * @brief The canonical alias of the room, if it exists. Otherwise falls back to the first available alias.
      */
-    readonly property var roomAlias: room.aliases[0]
+    readonly property string roomAlias: room?.aliases[0] ?? ""
 
-    Layout.fillWidth: true
+    spacing: 0
 
     RowLayout {
         Layout.fillWidth: true