🐛 fix(exam): correct profit calculation default values

- 将profitAmount、profitPercent、totalProfitAmount和totalProfitPercent的默认值从null改为0
- 修复提交记录中利润相关字段可能出现的null值问题，确保数值计算准确性

src/server/socket/services/exam.service.ts

@@ -221,10 +221,10 @@ export class ExamService {
         submissionRecord.holdingStock = answer.holdingStock || null;
         submissionRecord.holdingCash = answer.holdingCash || null;
         submissionRecord.price = Number(answer.price);
-        submissionRecord.profitAmount = answer.profitAmount || null;
-        submissionRecord.profitPercent = answer.profitPercent || null;
-        submissionRecord.totalProfitAmount = answer.totalProfitAmount || null;
-        submissionRecord.totalProfitPercent = answer.totalProfitPercent || null;
+        submissionRecord.profitAmount = answer.profitAmount || 0;
+        submissionRecord.profitPercent = answer.profitPercent || 0;
+        submissionRecord.totalProfitAmount = answer.totalProfitAmount || 0;
+        submissionRecord.totalProfitPercent = answer.totalProfitPercent || 0;
 
         recordsToSave.push(submissionRecord);
       }