Gui: modernize C++: return braced init list

src/Gui/QuantitySpinBox.cpp

@@ -314,7 +314,7 @@ QString QuantitySpinBox::boundToName() const
         std::string path = getPath().toString();
         return QString::fromStdString(path);
     }
-    return QString();
+    return {};
 }
 
 /**
@@ -377,7 +377,7 @@ QString Gui::QuantitySpinBox::expressionText() const
     catch (const Base::Exception& e) {
         qDebug() << e.what();
     }
-    return QString();
+    return {};
 }
 
 void QuantitySpinBox::evaluateExpression()