Fix parsing struct stats after schema evolution

[Tom-Newton]

Description

When a delta table's schema is evolved the struct stat schemas in checkpoints are also evolved. Since the struct stats are stored in a columnar way adding a single file with the new columns will cause nulls to appear in the struct stats for all other files. This is a significant difference compared to the json stats.

Unfortunately I overlooked this in #656 for both nullCounts and min/max values. This caused parsed struct stats to have extra columns full of nulls. I don't know if this was actually an issue at all but it should be fixed even if just for the sake of the warnings spam.

[2022-10-24T22:13:22Z WARN  deltalake::action::parquet_read] Expect type of nullCount field to be struct or int64, got: null
[2022-10-24T22:13:22Z WARN  deltalake::action::parquet_read] Expect type of nullCount field to be struct or int64, got: null
[2022-10-24T22:13:22Z WARN  deltalake::action::parquet_read] Expect type of nullCount field to be struct or int64, got: null
[2022-10-24T22:13:22Z WARN  deltalake::action::parquet_read] Expect type of nullCount field to be struct or int64, got: null
[2022-10-24T22:13:22Z WARN  deltalake::action::parquet_read] Expect type of nullCount field to be struct or int64, got: null

Related Issue(s)

• Relates to Can't utilise struct delta stats with delta_table.to_pyarrow_dataset() #653 but for the most part its an already solved issue.

Changes:

• Replace the test data with similar test data that includes a schema evolution.
• Add error handling for min/max values to ensure warnings will be logged for other unexpected types (there probably shouldn't be any). As a total rust noob I originally filled with nulls but I think that was a mistake.
• Ignore nulls for min/max stats and null count stats since these are expected after schema evolution and should be ignored without logging a warning.

Usual disclaimer on a PR from me: I don't know what I'm doing writing rust code. (thanks to wjones for tidying up my dodgy rust code 🙂)

[wjones127]

This is still a draft, but thought I might leave some early feedback.

[wjones127]

Thanks @Tom-Newton