Omit empty on optional column tags

worksheet.go

@@ -11,12 +11,12 @@ import (
 type xlsxWorksheet struct {
 	XMLName   xml.Name      `xml:"http://schemas.openxmlformats.org/spreadsheetml/2006/main worksheet"`
 	Dimension xlsxDimension `xml:"dimension"`
-	Cols      xslxCols      `xml:"cols"`
+	Cols      xslxCols      `xml:"cols,omitempty"`
 	SheetData xlsxSheetData `xml:"sheetData"`
 }
 
 type xslxCols struct {
-	Col []xlsxCol `xml:"col"`
+	Col []xlsxCol `xml:"col,omitempty"`
 }
 
 type xlsxCol struct {