fix: avoid returning nulls to prevent later NPE

Fixes #163

Author: mlopezFC

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) {

src/test/java/com/flowingcode/vaadin/addons/gridexporter/GridExporterMultipleHeaderRowsDemo.java

@@ -74,7 +74,7 @@ public GridExporterMultipleHeaderRowsDemo() throws EncryptedDocumentException, I
     @SuppressWarnings("null")
     BigDecimal total = people.stream().map(Person::getBudget).map(BigDecimal::valueOf)
         .reduce(BigDecimal.ZERO, BigDecimal::add);
-    budgetColumn.setFooter(new DecimalFormat(NUMBER_FORMAT_PATTERN).format(total));
+//    budgetColumn.setFooter(new DecimalFormat(NUMBER_FORMAT_PATTERN).format(total));
 
     grid.setItems(people);
     grid.setWidthFull();