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ARROW-10595: [Rust] Simplify inner loop of min/max kernels for non-null case

Keeping track of `has_value` is not needed when there are no nulls in the array, we can just pick the first item.

Micro benchmark results are a bit noisy, but the `min 512` seems to be consistently faster:

```
min 512                 time:   [442.25 ns 444.75 ns 448.21 ns]
                        change: [-34.516% -33.618% -32.659%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 18 outliers among 100 measurements (18.00%)
  8 (8.00%) high mild
  10 (10.00%) high severe
```

Closes #8668 from Dandandan/min_max

Authored-by: Heres, Daniel <danielheres@gmail.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>