Add unit test for TestGetCellStringFunc, and simplify code

Author: xuri

cell.go

@@ -1502,11 +1502,11 @@ func (f *File) getCellStringFunc(sheet, cell string, fn func(x *xlsxWorksheet, c
 	idx, found := sort.Find(len(ws.SheetData.Row), func(i int) int {
 		if ws.SheetData.Row[i].R == row {
 			return 0
-		} else if ws.SheetData.Row[i].R > row {
+		}
+		if ws.SheetData.Row[i].R > row {
 			return -1
-		} else {
-			return 1
 		}
+		return 1
 	})
 	if !found {
 		return "", nil

cell_test.go

@@ -1265,3 +1265,14 @@ func TestSetCellIntFunc(t *testing.T) {
 func TestSIString(t *testing.T) {
 	assert.Empty(t, xlsxSI{}.String())
 }
+
+func TestGetCellStringFunc(t *testing.T) {
+	f := NewFile()
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetData.Row = []xlsxRow{{R: 2}}
+	val, err := f.GetCellValue("Sheet1", "A1")
+	assert.Empty(t, val)
+	assert.NoError(t, err)
+	assert.NoError(t, f.Close())
+}