This fixes GHSA-g27h-8qhm-6pff, prevent panic on a worksheet with an empty merged cell reference (#2379)

- Update unit tests

Author: arpitjain099

cell.go

@@ -1657,6 +1657,9 @@ func (ws *xlsxWorksheet) mergeCellsParser(cell string) (string, error) {
 				_ = sortCoordinates(rect)
 				ws.MergeCells.Cells[i].rect = rect
 			}
+			if len(ws.MergeCells.Cells[i].rect) == 0 {
+				continue
+			}
 			if cellInRange([]int{col, row}, ws.MergeCells.Cells[i].rect) {
 				cell = strings.Split(ws.MergeCells.Cells[i].Ref, ":")[0]
 				break

merge_test.go

@@ -238,4 +238,10 @@ func TestMergeCellsParser(t *testing.T) {
 	ws := &xlsxWorksheet{MergeCells: &xlsxMergeCells{Cells: []*xlsxMergeCell{nil}}}
 	_, err := ws.mergeCellsParser("A1")
 	assert.NoError(t, err)
+
+	// Test parse merged cells with empty merged cell reference
+	ws = &xlsxWorksheet{MergeCells: &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: ""}}}}
+	cell, err := ws.mergeCellsParser("A1")
+	assert.NoError(t, err)
+	assert.Equal(t, "A1", cell)
 }