WriteEmptyStringAsNull implementation (#725)

Co-authored-by: Ivan Jiang <[email protected]>

Author: jiangyi1985

src/MiniExcel/OpenXml/ExcelOpenXmlSheetWriter.cs

@@ -338,7 +338,7 @@ private void WriteCell(MiniExcelStreamWriter writer, int rowIndex, int cellIndex
             }
 
             var columnReference = ExcelOpenXmlUtils.ConvertXyToCell(cellIndex, rowIndex);
-            var valueIsNull = value is null || value is DBNull;
+            var valueIsNull = value is null || value is DBNull || (_configuration.WriteEmptyStringAsNull && value is String && value == string.Empty);
 
             if (_configuration.EnableWriteNullValueCell && valueIsNull)
             {

src/MiniExcel/OpenXml/OpenXmlConfiguration.cs

@@ -13,6 +13,7 @@ public class OpenXmlConfiguration : Configuration
         public bool EnableConvertByteArray { get; set; } = true;
         public bool IgnoreTemplateParameterMissing { get; set; } = true;
         public bool EnableWriteNullValueCell { get; set; } = true;
+        public bool WriteEmptyStringAsNull { get; set; } = false;
         public bool EnableSharedStringCache { get; set; } = true;
         public long SharedStringCacheSize { get; set; } = 5 * 1024 * 1024;
         public DynamicExcelSheet[] DynamicSheets { get; set; }