fix: serde compatibility of null_count

[dantengsky]

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Summary

Fix fuse table meta data incompatibility introduced by PR #6067, by

• rename ColumnStatistics::unset_bits to ColumnStatistics:null_count
• and serde alias ColumnStatistics:null_count as ColumnStatistics::unset_bits

so that filed named null_count or unset_bits can both be de-serialized

Manually checked, that binary of this PR can read data generated by the commits that brought the incompatibility and the commit before that.

Namely, the following two commits.

commit e267f7ac91a169b5d9baa86311041ca94d5f503f
Merge: fef12ed99 ee005fa80
Author: BohuTANG <overred.shuttler@gmail.com>
Date:   Thu Jul 7 11:38:27 2022 +0800

    Merge pull request #6067 from dantengsky/feat-abandon-internal-parquet2-patches

    refactor: try abandon internal parquet2 patches

commit 19de25241095b0a3903dd0ca9f20f0ab60747db6
Author: lichuang <lichuang1982@gmail.com>
Date:   Thu Jul 7 11:33:58 2022 +0800

    fix: add watch txn unit test

Fixes #6556

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