Filter operators for null and notnull don't seem to work

[eddiesholl]

Describe the bug
I'm pretty sure the filter operators null and notnull aren't working. I don't think they are understood once they are passed through to mikroorm for execution.

The set of available files that should be supported are listed at https://graphweaver.com/docs/filters-and-operators
The mikroorm filter syntax docs are here https://mikro-orm.io/docs/query-conditions, and there's no mention of null handling

I have tried running a filter like { council_null: true } where council is a field on an entity, and it throws an error as mikroorm tries to prepare the SQL query

To Reproduce
Steps to reproduce the behavior:

1. Set up a query with a filter using a _null suffix
2. Execute the query
3. The query fails at runtime

Expected behavior
The documented filter operators should all be supported

Actual behavior
What actually happened.

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Desktop (please complete the following information):

• OS and version: [e.g. MacOS 13.3.1, Windows 11 v22H2]

Additional context
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