Applying filters with top-level checkboxes cause issues

[greatislander]

Describe the bug
Using hidden fields for the top-level topics (etc.) with an empty value unless checked breaks the query in WordPress by overwriting other values. We should leave the checkbox but hide it so as to ensure that it doesn't pass empty values with the form submission.

To Reproduce
Steps to reproduce the behavior:

1. Apply filters.
2. Observe empty values for unchecked top-level categories in query string.

Expected behavior
Query string only includes checked values.

Additional context
Not applicable.