Fix tuple append row handling to gracefully accept empty structs.

[jtmolon]

Summary

AppendRow fails with conversion error if the value of the tuple field is a nil pointer. This is a problem when inserting from protobuf using optional nested messages, when these are not given a value.

Example error:

clickhouse [AppendRow]: updated_ts clickhouse [AppendRow]: converting *timestamppb.Timestamp to Tuple(seconds Int64, nanos Int32) is unsupported

This change allows for graceful handling of empty structs. Full example here: https://gist.github.com/jtmolon/569f5a37bed17620c4c976baee0eb7ab

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

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