Rewrite equality with is null check in JOIN ON section

[vdimir]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• Rewrite equality with is null check in JOIN ON section. Analyzer only

SET allow_experimental_analyzer = 1;
SELECT * FROM t1 JOIN t2 ON a = b OR (a IS NULL AND b IS NULL)

will be transformed into

SELECT * FROM t1 JOIN t2 ON a <=> b

It provides better compatibility with BI tools.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[robot-ch-test-poll]

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[clickhouse-ci]

This is an automatic comment. The PR descriptions does not match the template.

Please, edit it accordingly.

The error is: Category 'Performance Optimization' is not valid

[vdimir]

I changed changelog entry from Performance Improvement back to Improvement because such expressions in JOIN ON was not supported at all and PR adds possibility to execute such a query, so it doesn't improve a query execution speed.
And in SELECT expression we don't support <=> operator, so no change there.