修复当cell为空可能会抛出空指针的bug

src/main/java/com/alibaba/excel/analysis/v07/handlers/DefaultCellHandler.java

@@ -7,6 +7,8 @@ import static com.alibaba.excel.constant.ExcelXmlConstants.CELL_TAG;
 import static com.alibaba.excel.constant.ExcelXmlConstants.CELL_VALUE_TAG;
 import static com.alibaba.excel.constant.ExcelXmlConstants.CELL_VALUE_TYPE_TAG;
 
+import java.util.Deque;
+import java.util.LinkedList;
 import java.util.Map;
 import java.util.TreeMap;
 
@@ -33,8 +35,7 @@ import com.alibaba.excel.util.StringUtils;
  */
 public class DefaultCellHandler implements XlsxCellHandler, XlsxRowResultHolder {
     private final AnalysisContext analysisContext;
-    private String currentTag;
-    private String currentCellIndex;
+    private Deque<String> currentTagDeque = new LinkedList<String>();
     private int curCol;
     private Map<Integer, CellData> curRowContent = new TreeMap<Integer, CellData>();
     private CellData currentCellData;
@@ -61,11 +62,10 @@ public class DefaultCellHandler implements XlsxCellHandler, XlsxRowResultHolder
 
     @Override
     public void startHandle(String name, Attributes attributes) {
-        currentTag = name;
+        currentTagDeque.push(name);
         // start a cell
         if (CELL_TAG.equals(name)) {
-            currentCellIndex = attributes.getValue(ExcelXmlConstants.POSITION);
-            curCol = PositionUtils.getCol(currentCellIndex);
+            curCol = PositionUtils.getCol(attributes.getValue(ExcelXmlConstants.POSITION));
 
             // t="s" ,it's means String
             // t="str" ,it's means String,but does not need to be read in the 'sharedStrings.xml'
@@ -100,6 +100,7 @@ public class DefaultCellHandler implements XlsxCellHandler, XlsxRowResultHolder
 
     @Override
     public void endHandle(String name) {
+        currentTagDeque.pop();
         if (CELL_VALUE_TAG.equals(name)) {
             // Have to go "sharedStrings.xml" and get it
             if (currentCellData.getType() == CellDataTypeEnum.STRING) {
@@ -133,6 +134,7 @@ public class DefaultCellHandler implements XlsxCellHandler, XlsxRowResultHolder
         if (StringUtils.isEmpty(currentCellValue)) {
             return;
         }
+        String currentTag = currentTagDeque.peek();
         if (currentTag == null) {
             return;
         }