Steps to reproduce

1. load a blank xlsx file
2. export it as a PDF like

   		File file = new File(currentFoler+ss.getBook().getBookName()+".pdf");
   		System.out.println(file.getAbsolutePath());
   		FileOutputStream fos = new FileOutputStream(file);
   		//shall get exporter for each exporting
   		Exporters.getExporter("pdf").export(ss.getBook(), fos);
   		Filedownload.save(file, "application/excel");
   

Actual Result

ExceptionConverter: java.io.IOException: The document has no pages.
at com.lowagie.text.pdf.PdfPages.writePageTree(Unknown Source)
at com.lowagie.text.pdf.PdfWriter.close(Unknown Source)
at com.lowagie.text.pdf.PdfDocument.close(Unknown Source)
at com.lowagie.text.Document.close(Unknown Source)
at org.zkoss.zss.model.impl.pdf.PdfExporter.export(PdfExporter.java:272)
at org.zkoss.zss.api.impl.ExporterImpl.export(ExporterImpl.java:49)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at bsh.Reflect.invokeMethod(Unknown Source)
at bsh.Reflect.invokeObjectMethod(Unknown Source)
at bsh.BSHPrimarySuffix.doName(Unknown Source)
at bsh.BSHPrimarySuffix.doSuffix(Unknown Source)
at bsh.BSHPrimaryExpression.eval(Unknown Source)
at bsh.BSHPrimaryExpression.eval(Unknown Source)
at bsh.BSHBlock.evalBlock(Unknown Source)
at bsh.BSHBlock.eval(Unknown Source)
at bsh.BshMethod.invokeImpl(Unknown Source)
at bsh.BshMethod.invoke(Unknown Source)
at bsh.BshMethod.invoke(Unknown Source)
at bsh.Name.invokeLocalMethod(Unknown Source)
at bsh.Name.invokeMethod(Unknown Source)
at bsh.BSHMethodInvocation.eval(Unknown Source)
at bsh.BSHPrimaryExpression.eval(Unknown Source)
at bsh.BSHPrimaryExpression.eval(Unknown Source)
at bsh.Interpreter.eval(Unknown Source)
at bsh.Interpreter.eval(Unknown Source)
at org.zkoss.zk.scripting.bsh.BSHInterpreter.exec(BSHInterpreter.java:133)
at org.zkoss.zk.scripting.util.GenericInterpreter.interpret(GenericInterpreter.java:328)
at org.zkoss.zk.ui.impl.PageImpl.interpret(PageImpl.java:900)

Expected result

a friendly message "Spreadsheet find nothing to print" or just export one blank page.