Parquet statistics stats_null_count is wrong for large json string colums

stats_null_count always None when one of the columns values is about 46977 characters length   

**Reporter**: [Rotem Levi](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=rotem.l)
#### Original Issue Attachments:
- [part-00000-7e5dcc44-d176-42ab-98b9-664cace5d433-c000.snappy.parquet](https://issues.apache.org/jira/secure/attachment/13058765/part-00000-7e5dcc44-d176-42ab-98b9-664cace5d433-c000.snappy.parquet)

_{**Note**: *This issue was originally created as [PARQUET-2306](https://issues.apache.org/jira/browse/PARQUET-2306). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

首先，使用附加的 part-00000-7e5dcc44-d176-42ab-98b9-664cace5d433-c000.snappy.parquet 文件以及包含大约 46,977 个字符的 JSON 字符串的列，重现报告的 stats_null_count 行为。跟踪该列的 Parquet 统计信息是如何读取的；当 stats_null_count 被正确报告而不是始终为 None 时，即表示完成。

由索引模型根据 Issue 内容生成。