fix: avoid setting thisSemesterStartDate when it is missing

[sseu-buhzzi]

For example this semester [1] does not have a start date specified in
the fetched data [2].

[1]

                  <option name="semesterAssoc" value="528">2025-2026学年暑期</option>

[2]

  var semesters = JSON.parse([
    {"startDate":"2026-03-01","endDate":"2026-07-04","name":"2025-2026学年2学期","id":505},
    {"startDate":"2025-09-07","endDate":"2026-01-10","name":"2025-2026学年1学期","id":504},
    {"startDate":"2025-06-22","endDate":"2025-09-06","name":"2024-2025学年暑期","id":506},
    {"startDate":"2025-02-16","endDate":"2025-06-21","name":"2024-2025学年2学期","id":469},
    {"startDate":"2024-09-01","endDate":"2025-01-04","name":"2024-2025学年1学期","id":467},
    {"startDate":"2024-02-25","endDate":"2024-06-29","name":"2023-2024学年2学期","id":464},
    {"startDate":"2023-09-03","endDate":"2024-01-06","name":"2023-2024学年1学期","id":444},
    {"startDate":"2023-02-19","endDate":"2023-06-24","name":"2022-2023学年2学期","id":425},
    {"startDate":"2022-09-04","endDate":"2023-01-07","name":"2022-2023学年1学期","id":404}
  ]);

[w568w]

Mostly LGTM. Just a minor change.

[w568w]

LGTM. Thanks for your contribution!