fix: 增强随机点名动画健壮性及错误处理

[seer-by-sentry]

Fixes SECRANDOM-2W. The issue was that: Unset default class causes draw_random_students to return early, leaving final_selected_students as None, leading to TypeError in create_student_label.

• 增加了对 final_selected_students 和 selected_students 为空或 None 的健壮性检查，避免潜在错误。
• 优化了随机点名动画在未抽取到学生或未设置默认班级时的处理逻辑，确保动画能正常停止并给出警告。
• 在未设置默认抽取名单或未抽取到学生时，增加了相应的日志警告。

This fix was generated by Seer in Sentry, triggered by wsxyt. 👁️ Run ID: 552657

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