fix: error when there is no unit bibliography

[limwa]

This PR fixes an issue where fetching a curricular unit's details fails when there is no bibliography on SIGARRA for the unit.

Review checklist

• [x] Terms and conditions reflect the current change
• [ ] Contains enough appropriate tests
• [x] If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• [x] Properly adds an entry in changelog.md with the change
• [x] If PR includes UI updates/additions, its description has screenshots
• [x] Behavior is as expected
• [x] Clean, well-structured code

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 11%. Comparing base (3909b97) to head (e0d8d97). Report is 7 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1323 +/- ## ======================================= + Coverage 11% 11% +1% ======================================= Files 268 268 Lines 7303 7302 -1 ======================================= Hits 799 799 + Misses 6504 6503 -1 ```