Empty Where Filter Input on Count Endpoints

[VKTB]

This PR will close #309

Description

The changes in this PR make the count endpoints work when they receive an empty where filter input (where={}). Also added some extra code so that FilterError is raised when the where filter input is not an object.

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?

[VKTB]

The failing test jobs will be fixed by #325

[MRichards99]

Looks good, just need to reword the error message as per my comment and merge master in once #325 is merged so the CI passes.

[codecov]

Codecov Report

Merging #326 (e23f0c4) into master (80222ee) will increase coverage by 0.00%.
The diff coverage is 93.54%.

@@           Coverage Diff           @@
##           master     #326   +/-   ##
=======================================
  Coverage   92.62%   92.62%           
=======================================
  Files          43       43           
  Lines        3199     3242   +43     
  Branches      318      325    +7     
=======================================
+ Hits         2963     3003   +40     
- Misses        198      200    +2     
- Partials       38       39    +1     

Impacted Files	Coverage Δ
datagateway_api/src/search_api/helpers.py	97.53% <ø> (-2.47%)	⬇️
...gateway_api/src/search_api/query_filter_factory.py	95.65% <93.54%> (+0.09%)	⬆️
datagateway_api/src/search_api/filters.py	89.15% <0.00%> (-0.46%)	⬇️
...agateway_api/src/resources/search_api_endpoints.py	100.00% <0.00%> (ø)
datagateway_api/src/search_api/models.py	96.69% <0.00%> (+0.16%)	⬆️
datagateway_api/src/api_start_utils.py	86.29% <0.00%> (+0.45%)	⬆️

---

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 80222ee...e23f0c4. Read the comment docs.

[MRichards99]

Looks good, we should probably wait to merge this PR so we can keep one in reserve for the DOI minting discussed on Slack.

[MRichards99]

This was waiting for a DOI to be minted for the search API. I believe this is unlikely to happen now, so the merge conflicts should be resolved and the PR should be merged.