This closes #756, not set the empty string for the cell when SetCellValue with nil

cell.go

@@ -93,7 +93,7 @@ func (f *File) SetCellValue(sheet, axis string, value interface{}) error {
 	case bool:
 		err = f.SetCellBool(sheet, axis, v)
 	case nil:
-		err = f.SetCellStr(sheet, axis, "")
+		break
 	default:
 		err = f.SetCellStr(sheet, axis, fmt.Sprint(value))
 	}

cell_test.go

@@ -331,7 +331,7 @@ func TestFormattedValue2(t *testing.T) {
 	f.Styles.CellXfs.Xf = append(f.Styles.CellXfs.Xf, xlsxXf{
 		NumFmtID: nil,
 	})
-	v = f.formattedValue(3, "43528")
+	_ = f.formattedValue(3, "43528")
 
 	// formatted value with empty number format
 	f.Styles.NumFmts = nil

drawing.go

@@ -844,7 +844,7 @@ func (f *File) drawChartSeriesVal(v formatChartSeries, formatSet *formatChart) *
 // drawChartSeriesMarker provides a function to draw the c:marker element by
 // given data index and format sets.
 func (f *File) drawChartSeriesMarker(i int, formatSet *formatChart) *cMarker {
-	defaultSymbol := map[string]*attrValString{Scatter: &attrValString{Val: stringPtr("circle")}}
+	defaultSymbol := map[string]*attrValString{Scatter: {Val: stringPtr("circle")}}
 	marker := &cMarker{
 		Symbol: defaultSymbol[formatSet.Type],
 		Size:   &attrValInt{Val: intPtr(5)},

xmlChart.go

@@ -138,25 +138,25 @@ type aSchemeClr struct {
 }
 
 // attrValInt directly maps the val element with integer data type as an
-// attribute。
+// attribute.
 type attrValInt struct {
 	Val *int `xml:"val,attr"`
 }
 
 // attrValFloat directly maps the val element with float64 data type as an
-// attribute。
+// attribute.
 type attrValFloat struct {
 	Val *float64 `xml:"val,attr"`
 }
 
 // attrValBool directly maps the val element with boolean data type as an
-// attribute。
+// attribute.
 type attrValBool struct {
 	Val *bool `xml:"val,attr"`
 }
 
 // attrValString directly maps the val element with string data type as an
-// attribute。
+// attribute.
 type attrValString struct {
 	Val *string `xml:"val,attr"`
 }