Support Is /not Null filter for value is None on webui

[chenglongyan]

Snapshot:

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closes: #26427
related: #26461

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^ Add meaningful description above

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[potiuk]

looks like some tests are failing?

[ashb]

From a "SQL" point of view "" (emptry string) is different to None, (SQL NULL). As you've written your patch it's not possible to find values with an empty string.

Is it worth adding two extra filters for "is null" and "is not null"?

[chenglongyan]

Agreed, there are ambiguities...

From a "SQL" point of view "" (emptry string) is different to None, (SQL NULL).