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

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

[codecov]

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

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