Fix crash in topK when merging empty states

[Algunenano]

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 to CHANGELOG.md):

• Fix crash in topK when merging empty states

Closes #59107

[robot-ch-test-poll1]

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Check name	Description	Status
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Check name	Description	Status
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Integration tests	The integration tests report. In parenthesis the package type is given, and in square brackets are the optional part/total tests	❌ failure
Stateless tests	Runs stateless functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	❌ failure