Antalya 25.8 backport of #84770: Handle NULLs in ALTER MODIFY COLUMN

[mkmkme]

25.8.15 Stable backport of ClickHouse#84770: Handle NULLs in ALTER MODIFY COLUMN

Same as #1344, but for Antalya 25.8

Changelog category (leave one):

• Backward Incompatible Change

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

ALTER MODIFY COLUMN now requires explicit DEFAULT when converting nullable columns to non-nullable types. Previously such ALTERs could get stuck with cannot convert null to not null errors, now NULLs are replaced with column's default expression (ClickHouse#84770 by @vdimir)

Documentation entry for user-facing changes

...

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• Aarch64 tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• OAuth (5m)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)

[mkmkme]

The tests passed locally. Let's see if something breaks in CI. After that I'll self-verify

[github-actions]

Workflow [PR], commit [9c9555d]