Don't load empty images from imageprovider

Previously, when there was no avatar set, the source property of Avatar was still set to 'image://mxc/',
which caused Avatar to load that from the imageprovider. The imageprovider can't provide an empty image and aborts with error


(cherry picked from commit 724f10a895)

imports/NeoChat/Page/RoomListPage.qml

@@ -134,7 +134,7 @@ Kirigami.ScrollablePage {
                     Layout.minimumWidth: size
                     Layout.maximumWidth: size
 
-                    source: avatar ? "image://mxc/" + avatar : ""
+                    source: avatar ? ("image://mxc/" + avatar) : ""
                     name: model.name || i18n("No Name")
                 }