`Converter`支持null转换 [Issue #1776]

src/main/java/com/alibaba/excel/write/executor/AbstractExcelWriteExecutor.java

@@ -73,8 +73,7 @@ public abstract class AbstractExcelWriteExecutor implements ExcelWriteExecutor {
     }
 
     protected CellData<?> convert(WriteHolder currentWriteHolder, Class<?> clazz, CellDataTypeEnum targetType,
-        Cell cell,
+        Cell cell, Object value, ExcelContentProperty excelContentProperty) {
-        Object value, ExcelContentProperty excelContentProperty) {
         // This means that the user has defined the data.
         if (value instanceof CellData) {
             CellData<?> cellDataValue = (CellData<?>)value;