Fix reading null map subcolumn from Variants that cannot be inside Nullable

[Avogar]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

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

Fix reading null map subcolumn from Variants that cannot be inside Nullable.

Closes #87710

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[clickhouse-gh]

Workflow [PR], commit [c4edcec]

Summary: ❌

job_name	test_name	status	info	comment
Integration tests (amd_tsan, 4/6)		failure
	test_replicated_merge_tree_encryption_codec/test.py::test_same_keys	FAIL
Integration tests (amd_tsan, 6/6)		failure
	test_throttling/test.py::test_remote_write_throttling_reload	FAIL

[rienath]

@Avogar I left one comment. Also, there's a failing test that is trying to extract .null from a variant without a Nullable inside, which is now illegal. Otherwise looks good!