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/JoinRoomPage.qml

@@ -116,7 +116,7 @@ Kirigami.ScrollablePage {
                     Layout.preferredWidth: Kirigami.Units.iconSizes.huge
                     Layout.preferredHeight: Kirigami.Units.iconSizes.huge
 
-                    source: model.avatar ? "image://mxc/" + model.avatar : ""
+                    source: model.avatar ? ("image://mxc/" + model.avatar) : ""
                     name: name
                 }
                 ColumnLayout {