basic: fix for UnoControlTabPageModel get/set properties

The userformscontainers is required property to pass checks in
getter/setter, but returning true instead of actual type is
not a best idea. So let's return actually expected dummy empty
container.

Change-Id: I5cc3e5462ed82f6f2f8e5a45d9fc2d9f9ce1c76f
Reviewed-on: https://gerrit.libreoffice.org/67431
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

toolkit/source/controls/tabpagemodel.cxx

@@ -86,9 +86,10 @@ Any UnoControlTabPageModel::ImplGetDefaultValue( sal_uInt16 nPropId ) const
             break;
         case BASEPROPERTY_USERFORMCONTAINEES:
         {
-            // We do not have here any usercontainers (yet?), but let's return something back
+            // We do not have here any usercontainers (yet?), but let's return empty container back
             // so normal properties could be set without triggering UnknownPropertyException
-            return makeAny(true);
+            aAny <<= uno::Reference< XNameContainer >();
+            break;
         }
         default:
             aAny = UnoControlModel::ImplGetDefaultValue( nPropId );