fix oob if in .get

[ritchie46]

if the array doesn't have a validity is_null would not panic on OOB, leading to wrong assumptions in get that could lead to OOB.

[codecov[bot]]

Codecov Report

Patch coverage: 83.33% and no project coverage change.

Comparison is base (92050ec) 83.44% compared to head (1041ffd) 83.44%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1529 +/- ## ======================================= Coverage 83.44% 83.44% ======================================= Files 388 388 Lines 42137 42141 +4 ======================================= + Hits 35161 35165 +4 Misses 6976 6976 ``` | [Files Changed](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1529?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao) | Coverage Δ | | |---|---|---| | [src/io/ipc/read/file.rs](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1529?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL2lvL2lwYy9yZWFkL2ZpbGUucnM=) | `95.59% <0.00%> (ø)` | | | [src/array/mod.rs](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1529?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL2FycmF5L21vZC5ycw==) | `79.72% <100.00%> (+0.28%)` | :arrow_up: | ... and [4 files with indirect coverage changes](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1529/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao)

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