v.25.6Performance Improvement

Introduced a happy path in UniqExactSet::merge

Official CH changelog text
Introduced a happy path in UniqExactSet::merge when one of the sets is empty. Also, now if the LHS set is two-level and the RHS is single-level, we won't do the conversion to two-level for the RHS. #79971 (Nikita Taranov).
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