QmlDesigner: Fix compile with gcc 5.3

Amends c12e3d58ee.

Change-Id: Iae593a499153c772751c3d4acfde43518ece981f
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/assetexporterplugin/parsers/textnodeparser.cpp

@@ -42,7 +42,7 @@ const QHash<QString, QString> AlignMapping{
 
 QString toJsonAlignEnum(QString value) {
     if (value.isEmpty() || !AlignMapping.contains(value))
-        return "";
+        return {};
     return AlignMapping[value];
 }
 }