fix: avoid returning nulls to prevent later NPE

Fixes #163

src/main/java/com/flowingcode/vaadin/addons/gridexporter/BaseStreamResourceWriter.java

@@ -162,7 +162,7 @@ private String renderCellTextContent(Grid<T> grid, Column<T> column, String colu
       }
     }
 
-    return headerOrFooter;
+    return headerOrFooter==null?"":headerOrFooter;
   }
 
   protected Stream<T> obtainDataStream(DataProvider<T, ?> dataProvider) {