Exporting fails when there are no footers attached

[brunoagretti]

Describe the bug

When exporting as .xlsx a grid which has no footers added, the following exception is thrown, and the download fails:

Caused by: javax.servlet.ServletException: com.vaadin.flow.server.ServiceException: java.lang.NullPointerException
	at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:371)
	at com.vaadin.flow.spring.SpringServlet.service(SpringServlet.java:106)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:623)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:209)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)
	at vaadin.hibernate.CurrentServerContextFilter.doFilterInternal(CurrentServerContextFilter.java:163)
	... 48 more
Caused by: com.vaadin.flow.server.ServiceException: java.lang.NullPointerException
	at com.vaadin.flow.server.VaadinService.handleExceptionDuringRequest(VaadinService.java:1609)
	at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1572)
	at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:369)
	... 56 more
Caused by: java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:209)
	at java.base/java.util.ImmutableCollections$List12.<init>(ImmutableCollections.java:556)
	at java.base/java.util.List.of(List.java:812)
	at com.flowingcode.vaadin.addons.gridexporter.ExcelStreamResourceWriter.lambda$fillFooter$4(ExcelStreamResourceWriter.java:430)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
	at com.flowingcode.vaadin.addons.gridexporter.ExcelStreamResourceWriter.fillFooter(ExcelStreamResourceWriter.java:430)
	at com.flowingcode.vaadin.addons.gridexporter.ExcelStreamResourceWriter.createWorkbook(ExcelStreamResourceWriter.java:128)
	at com.flowingcode.vaadin.addons.gridexporter.ExcelStreamResourceWriter.accept(ExcelStreamResourceWriter.java:79)
	at com.flowingcode.vaadin.addons.gridexporter.ConcurrentStreamResourceWriter.accept(ConcurrentStreamResourceWriter.java:254)
	at com.vaadin.flow.server.communication.StreamResourceHandler.handleRequest(StreamResourceHandler.java:86)
	at com.vaadin.flow.server.communication.StreamRequestHandler.handleRequest(StreamRequestHandler.java:110)
	at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1560)
	... 57 more

Moreover, grids that do have a footer attached can be exported correctly.

Expected behavior

No response

Minimal reproducible example

No response

Add-on Version

1.10.0-SNAPSHOT

Vaadin Version

23.3.32

Additional information

No response