[DeckShare] Explain why a blank deck cannot be shared

A blank deck exited the share flow silently. The menu only disables the entry
via setSaveStatus(), a different predicate, so the path is reachable (e.g. add a
card and remove it again). Mirror the not-logged-in branch with a short
information dialog.

cockatrice/src/interface/widgets/tabs/abstract_tab_deck_editor.cpp

@@ -395,6 +395,7 @@ void AbstractTabDeckEditor::actShareDeck()
 
     const QSharedPointer<DeckList> deck = deckStateManager->getDeckListShared();
     if (deck->isBlankDeck()) {
+        QMessageBox::information(this, tr("Share deck"), tr("The deck is empty. Add cards before sharing it."));
         return;
     }