Fix parsing null counts for struct type columns in the struct stats

[Tom-Newton]

Description

When reading the struct column stats (as opposed to json column stats) the null counts where ignored for struct type columns. Completely ignoring is probably less bad than parsing them wrongly but if the user was filtering on a struct column this may have impacted performance. It was also spamming error logs:

[2022-07-26T19:33:52Z ERROR deltalake::action] Expect type of stats_parsed.nullRecords value to be struct, got: {integer: 0, null: 1, boolean: 0, double: 0, decimal: 0, string: 0, binary: 0, date: 0, timestamp: 0, struct: {struct_element: 0}, map: 0, array: 0, nested_struct: {struct_element: {nested_struct_element: 0}}, struct_of_array_of_map: {struct_element: 0}}

I probably should have done this as part of #656. Sorry for introducing this slight regression. I think some logging was getting suppressed during the unittest so I didn't notice it was an issue.

Related Issue(s)

Relates to #653 but that was for the most part already solved by #656

Changes

• In the test assert that all stats are the same not just the min_values stats. This prevents a similar mistake in future.
• Updated logic to handle struct types when parsing null_counts from struct stats.

[Tom-Newton]

Unfortunately the PyArrow 4.0.0 build is failing

-- Could NOT find Arrow (missing: Arrow_DIR)
      -- Checking for module 'arrow'
      --   No package 'arrow' found
      CMake Error at /opt/_internal/pipx/venvs/cmake/lib/python3.9/site-packages/cmake/data/share/cmake-3.22/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
        Could NOT find Arrow (missing: ARROW_INCLUDE_DIR ARROW_LIB_DIR
        ARROW_FULL_SO_VERSION ARROW_SO_VERSION)
      Call Stack (most recent call first):
        /opt/_internal/pipx/venvs/cmake/lib/python3.9/site-packages/cmake/data/share/cmake-3.22/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
        cmake_modules/FindArrow.cmake:450 (find_package_handle_standard_args)
        cmake_modules/FindArrowPython.cmake:46 (find_package)
        CMakeLists.txt:236 (find_package)

https://github.com/delta-io/delta-rs/runs/7705681944?check_suite_focus=true
Though I think this is probably not caused by my changes?

[houqp]

@Tom-Newton it's not related to your change, but caused by the new arrow 9 release, see #724 (comment). You can ignore it for now and we can handle the fix in a different PR.