fix: 修复判断题答案为 false 时显示"暂无"的问题

当判断题的正确答案为 false（错误）时，由于 JavaScript 中 false 是假值，
使用 || 运算符会导致跳过该值，从而在用户答错时显示"暂无"。

修复方法：明确检查 correct_answer 是否为 undefined 或 null，
而不是依赖 || 运算符的短路逻辑。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

web/src/components/AnswerResult.tsx

@@ -180,7 +180,9 @@ const AnswerResult: React.FC<AnswerResultProps> = ({
                 </Text>
                 <Text strong style={{ color: '#52c41a' }}>
                   {formatAnswer(
-                    answerResult.correct_answer || (answerResult.correct ? selectedAnswer : '暂无')
+                    answerResult.correct_answer !== undefined && answerResult.correct_answer !== null
+                      ? answerResult.correct_answer
+                      : (answerResult.correct ? selectedAnswer : '暂无')
                   )}
                 </Text>
               </div>