QmlDesigner: Optimize method by using constinit QString for null return value

- Introduced constinit QString null to replace temporary QString object.
- Updated return type to const QString& for better performance and
consistency.
- Ensured method returns null when no valid binding property is found.

Change-Id: Ib74d45237d9396cca72d15bd550b6fb376da84cd
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/libs/designercore/include/bindingproperty.h

@@ -16,7 +16,7 @@ class QMLDESIGNERCORE_EXPORT BindingProperty final : public QmlDesigner::Abstrac
 
 public:
     void setExpression(const QString &expression);
-    QString expression() const;
+    const QString &expression() const;
 
     BindingProperty();
     BindingProperty(const BindingProperty &property, AbstractView *view);

src/plugins/qmldesigner/libs/designercore/model/bindingproperty.cpp

@@ -56,14 +56,16 @@ void BindingProperty::setExpression(const QString &expression)
     privateModel()->setBindingProperty(internalNodeSharedPointer(), name(), expression);
 }
 
-QString BindingProperty::expression() const
+const constinit QString null;
+
+const QString &BindingProperty::expression() const
 {
     if (isValid()) {
         if (auto property = internalNode()->bindingProperty(name()))
             return property->expression();
     }
 
-    return QString();
+    return null;
 }
 
 ModelNode BindingProperty::resolveBinding(const QString &binding, ModelNode currentNode) const

src/plugins/qmldesigner/libs/designercore/model/internalbindingproperty.cpp

@@ -17,7 +17,7 @@ bool InternalBindingProperty::isValid() const
     return InternalProperty::isValid() && isBindingProperty();
 }
 
-QString InternalBindingProperty::expression() const
+const QString &InternalBindingProperty::expression() const
 {
     return m_expression;
 }

src/plugins/qmldesigner/libs/designercore/model/internalbindingproperty.h

@@ -18,7 +18,7 @@ public:
 
     bool isValid() const override;
 
-    QString expression() const;
+    const QString &expression() const;
     void setExpression(const QString &expression);
 
     void setDynamicExpression(const TypeName &type, const QString &expression);