Don't generate externalReferences tags if non externalLinks exists

src/xlsx/xlsxworkbook.cpp

@@ -437,6 +437,7 @@ void Workbook::saveToXmlFile(QIODevice *device) const
     }
     writer.writeEndElement();//sheets
 
+    if (d->externalLinks.size() > 0) {
         writer.writeStartElement(QStringLiteral("externalReferences"));
         for (int i=0; i<d->externalLinks.size(); ++i) {
             writer.writeEmptyElement(QStringLiteral("externalReference"));
@@ -444,6 +445,7 @@ void Workbook::saveToXmlFile(QIODevice *device) const
             writer.writeAttribute(QStringLiteral("r:id"), QStringLiteral("rId%1").arg(d->relationships->count()));
         }
         writer.writeEndElement();//externalReferences
+    }
 
     if (!d->definedNamesList.isEmpty()) {
         writer.writeStartElement(QStringLiteral("definedNames"));