Use a disabled rectangle instead of an empty space in the XRay view

To keep consistency with what happens in the prepare page when a printer doesn't have material or variants.

Contributes to CURA-6469.

resources/qml/EmptyViewMenuComponent.qml

@@ -3,7 +3,26 @@
 
 import QtQuick 2.10
 import QtQuick.Controls 2.3
+import QtGraphicalEffects 1.0 // For the dropshadow
 
+import UM 1.2 as UM
 
 // Empty placeholder
-Item { }
+Rectangle
+{
+    color: UM.Theme.getColor("disabled")
+
+    DropShadow
+    {
+        id: shadow
+        // Don't blur the shadow
+        radius: 0
+        anchors.fill: parent
+        source: parent
+        verticalOffset: 2
+        visible: true
+        color: UM.Theme.getColor("action_button_shadow")
+        // Should always be drawn behind the background.
+        z: parent.z - 1
+    }
+}