Support for Empty Parts in MergeTree Engines

Why it matters

This feature addresses the issue where merges in the specified MergeTree engines could result in empty parts being discarded or causing errors. By supporting empty parts after merges, it ensures consistent and correct handling of data states where data removal results in empty parts, improving stability and correctness for use cases involving data deletion or collapsing.

How to use it

No special configuration is needed to enable this feature. Users benefit from improved handling of merges resulting in empty parts simply by upgrading to the version that includes this fix. Existing tables using SummingMergeTree, CollapsingMergeTree, and VersionedCollapsingMergeTree will handle empty parts correctly after merges.