fix NPE processing excel workbooks with empty worksheets

java.lang.NullPointerException: Cannot invoke method collectEntries() on null object
    at com.commercehub.griddle.poi.SheetTabularData.<init>(SheetTabularData.groovy:18)
    at com.commercehub.griddle.poi.WorkbookTabularDataSource.withFile(WorkbookTabularDataSource.groovy:21)
build: apply codenarc