[Feature Request] isNull filter

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Environment Setup

Steps to reproduce

I've seen this issue posted before a few years ago so this may be a duplicate. I noticed that null is showing up in my database.

In the database it show null as well

_id: "eKg7Ib3Lsx"
startDate: 2021-02-14T20:52:01.761+00:00
endDate: null
_created_at: 2021-02-14T20:52:02.467+00:00
_updated_at: 2021-02-22T18:04:06.333+00:00

I think we should add an isNull filter to find these values.

Logs/Trace

Note: If you get a browser JS error please run npm run dev. This will provide source maps and a much more useful stack trace.

The issue names no source file or test; start by reproducing the missing null-value filtering in Parse Dashboard, using npm run dev if browser errors need source maps. Trace the existing filter controls and query handling, then confirm that an isNull filter can find records whose field value is null.