Don't create the default "Sheet1" worksheet

Unless the user want to access it, or the user want to save an empty
workbook.

src/xlsx/xlsxdocument.cpp

@@ -96,9 +96,6 @@ void DocumentPrivate::init()
 
     if (workbook.isNull())
         workbook = QSharedPointer<Workbook>(new Workbook(Workbook::F_NewFromScratch));
-
-    if (workbook->sheetCount() == 0)
-        workbook->addSheet();
 }
 
 bool DocumentPrivate::loadPackage(QIODevice *device)

src/xlsx/xlsxworkbook.cpp

@@ -255,6 +255,8 @@ AbstractSheet *Workbook::insertSheet(int index, const QString &name, AbstractShe
 AbstractSheet *Workbook::activeSheet() const
 {
     Q_D(const Workbook);
+    if (d->sheets.isEmpty())
+        const_cast<Workbook*>(this)->addSheet();
     return d->sheets[d->activesheetIndex].data();
 }
 
@@ -426,6 +428,8 @@ void Workbook::saveToXmlFile(QIODevice *device) const
 {
     Q_D(const Workbook);
     d->relationships->clear();
+    if (d->sheets.isEmpty())
+        const_cast<Workbook *>(this)->addSheet();
 
     QXmlStreamWriter writer(device);