Empty Dict on Query Param for Count Endpoints

[MRichards99]

Description:
On count endpoints, the following query param doesn't work (400 is returned):
where={}

However, on non-count endpoints, the following query param does work and basically behaves in a way that there are no filters:
filter={}

We should make the count version have a similar behaviour as the non-count version.

I only noticed this while writing tests with pytest.param(), trying to reduce duplication. I don't know how much a user would use an empty dict but perhaps they might. This might be a case of fixing if we have time, but not stressing about it if we can't. If so, the tests that I've skipped can be easily rewritten in a number of minutes to avoid this bug.

Acceptance criteria:

• Make where={} work on count endpoints
• Unskip relevant tests in test_count_dataset_files.py and test_count_endpoint.py