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

imports/NeoChat/Panel/RoomDrawer.qml

@@ -105,7 +105,7 @@ Kirigami.OverlayDrawer {
                         Layout.preferredHeight: Kirigami.Units.gridUnit * 3.5
 
                         name: room ? room.displayName : i18n("No name")
-                        source: room ? "image://mxc/" +  room.avatarMediaId : undefined
+                        source: room ? ("image://mxc/" +  room.avatarMediaId) : ""
                     }
 
                     ColumnLayout {
@@ -184,7 +184,7 @@ Kirigami.OverlayDrawer {
                             Layout.preferredWidth: height
                             Layout.fillHeight: true
 
-                            source: "image://mxc/" + avatar
+                            source: avatar ? ("image://mxc/" + avatar) : ""
                             name: name
                         }