fix(course): make exam info field nullable

[CutebreadCat]

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这个 PR 的类型是什么？

fix

这个 PR 做了什么 / 我们为什么需要这个 PR？

修复了exam_info_sha256字段 null与json的问题

(可选)这个 PR 解决了哪个/些 issue？

对 Reviewer 预留的一些提醒

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/course/service/get_course_list.go	60.00%	1 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
- Coverage   65.49%   64.10%   -1.39%     
==========================================
  Files         241      241              
  Lines        6529     6581      +52     
==========================================
- Hits         4276     4219      -57     
- Misses       2095     2204     +109     
  Partials      158      158              

Flag	Coverage Δ
unittest	64.10% <60.00%> (-1.39%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/course/service/get_course_list.go	75.75% <60.00%> (-0.54%)	⬇️

... and 3 files with indirect coverage changes

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