fix: fix NPE when the grid to export does not contain a footer row

Fixes #20

Author: mlopezFC

pom.xml

@@ -155,7 +155,7 @@
 		<dependency>
 			<groupId>org.vaadin.addons.flowingcode</groupId>
 			<artifactId>grid-helpers</artifactId>
-			<version>0.2.0</version>
+			<version>0.3.1</version>
 		</dependency>
 		<dependency>
 			<groupId>org.slf4j</groupId>