Fix AttributeError when trying to run DG API on its own

[VKTB]

This PR will close #345

Testing Instructions

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?
• DG API can run on its own

[codecov]

Codecov Report

Merging #346 (7479cc3) into master (60a599e) will decrease coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
- Coverage   94.45%   94.42%   -0.03%     
==========================================
  Files          39       39              
  Lines        3065     3067       +2     
  Branches      306      307       +1     
==========================================
+ Hits         2895     2896       +1     
  Misses        144      144              
- Partials       26       27       +1     

Impacted Files	Coverage Δ
datagateway_api/src/common/filter_order_handler.py	98.61% <80.00%> (-1.39%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 60a599e...7479cc3. Read the comment docs.