[client-v2] Fixed reusing map without creating new nodes

[chernser]

Summary

Internally data is read into a map that represents a record. clear() was called each time before reading data to protect from stale data from a previous records. However it is not needed because it is always same number of columns are read. Only case with null should be handled correctly to clean up non-null values.

This single change reduces number of Map$Node object by magnitude.

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials