sync get_selected_id with get_active_id

Change-Id: I983cc45d13e02a020f608b8e9abfd2f12f7b10f5
Reviewed-on: https://gerrit.libreoffice.org/68741Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

include/vcl/weld.hxx

@@ -608,7 +608,11 @@ public:
     //by id
     virtual OUString get_id(int pos) const = 0;
     virtual int find_id(const OUString& rId) const = 0;
-    OUString get_selected_id() const { return get_id(get_selected_index()); }
+    OUString get_selected_id() const
+    {
+        int pos = get_selected_index();
+        return pos == -1 ? OUString() : get_id(pos);
+    }
     void select_id(const OUString& rId) { select(find_id(rId)); }
 
     //via iter