QJsonDocument: enable NRVO for gcc in toJson()

Change-Id: I1b639272d38f8463b17a85a406addb74bb572756
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/json/qjsondocument.cpp

@@ -351,10 +351,9 @@ QByteArray QJsonDocument::toJson() const
 #ifndef QT_JSON_READONLY
 QByteArray QJsonDocument::toJson(JsonFormat format) const
 {
-    if (!d)
-        return QByteArray();
-
     QByteArray json;
+    if (!d)
+        return json;
 
     if (d->header->root()->isArray())
         QJsonPrivate::Writer::arrayToJson(static_cast<QJsonPrivate::Array *>(d->header->root()), json, 0, (format == Compact));