fix(optimiser): Add default slots to unallocated lessons

[thejus03]

Context

• Fixes Optimiser partial timetable does not show unallocated lessons #4145

Implementation

• Added default slots for unassigned lessons

[vercel]

@thejus03 is attempting to deploy a commit to the modsbot's projects Team on Vercel.

A member of the Team first needs to authorize it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.16%. Comparing base (988c6fd) to head (5cefc73).
⚠️ Report is 150 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4264      +/-   ##
==========================================
+ Coverage   54.52%   55.16%   +0.63%     
==========================================
  Files         274      297      +23     
  Lines        6076     6729     +653     
  Branches     1455     1619     +164     
==========================================
+ Hits         3313     3712     +399     
- Misses       2763     3017     +254     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.