generate empty workbook if no data to add

cmd/main/main.go

@@ -153,6 +153,16 @@ func main() {
 	// Get an interface that we can work with to access the sub elements
 	vslice := vislice.([]interface{})
 
+	if len(vslice) < 1 {
+		// There was no data but lets just log that and close off the empty workbook
+		fmt.Printf("No data found contained in top-level json key '%s', no work to do.\n", parentNode)
+		// Close off the file
+		if err := xlsx.SaveAs(outputFilename); err != nil {
+			log.Fatal(err)
+		}
+		return
+	}
+
 	// Check that the first element is what we expected
 	if _, ok := vslice[0].(orderedmap.OrderedMap); !ok {
 		error := fmt.Sprintf("Type of first vslice element is not an ordered map. It appears to be '%v'\n", reflect.TypeOf(vslice[0]))