add an isEmpty method to printing info (#6805)

no reason, I just find it easier to understand

libcockatrice_card/libcockatrice/card/database/card_database_querier.cpp

@@ -133,7 +133,7 @@ ExactCard CardDatabaseQuerier::getRandomCard() const
 ExactCard CardDatabaseQuerier::getCardFromSameSet(const QString &cardName, const PrintingInfo &otherPrinting) const
 {
     // The source card does not have a printing defined, which means we can't get a card from the same set.
-    if (otherPrinting == PrintingInfo()) {
+    if (otherPrinting.isEmpty()) {
         return getCard({cardName});
     }
 
@@ -360,4 +360,4 @@ QMap<QString, int> CardDatabaseQuerier::getAllFormatsWithCount() const
     }
 
     return formatCounts;
-}
+}

libcockatrice_card/libcockatrice/card/printing/printing_info.h

@@ -54,6 +54,16 @@ public:
         return this->set == other.set && this->properties == other.properties;
     }
 
+    /**
+     * @brief check if the info is empty, as if default constructed.
+     *
+     * @return True if both set and properties are empty, otherwise false.
+     */
+    bool isEmpty() const
+    {
+        return set == nullptr && properties.isEmpty();
+    }
+
 private:
     CardSetPtr set;          ///< The set this variation belongs to.
     QVariantHash properties; ///< Key-value store for variation-specific attributes.