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/Dialog/UserDetailDialog.qml

@@ -51,7 +51,7 @@ Kirigami.OverlaySheet {
                 Layout.preferredHeight: Kirigami.Units.iconSizes.huge
 
                 name: displayName
-                source: avatarMediaId ? "image://mxc/" + avatarMediaId : ""
+                source: avatarMediaId ? ("image://mxc/" + avatarMediaId) : ""
 
                 MouseArea {
                     anchors.fill: parent