Fix IS NULL rendering of keyset queries using Querydsl

[stupid58fly]

This PR fixes the issue for querydsl scroll where one of keyset position values is null.
Before fix querydsl generates query with column = null and after fix it will be column is null.

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).

[mp911de]

Can we have simpler tests, ideally that demonstrate the problem and test the query itself rather than the effect?

[stupid58fly]

Can we have simpler tests, ideally that demonstrate the problem and test the query itself rather than the effect?

I tried to write this test, but didn't succeed.
There is no such tests for QueryDSL, at least I could not find it.
I have tried to do that, but couldn't find any point to join the query rendering.
Not sure it is possible to implement such test without refactoring QueryDSL implementation.
If you have any suggestions, could you please share?

[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.