ARROW-12015: [Rust] Fix Clippy

[alamb]

Master has some lint failures:
https://github.com/apache/arrow/pull/9749/checks?check_run_id=2144048180

error: this `else { if .. }` block can be collapsed
   --> arrow/src/array/array_binary.rs:427:20
    |
427 |               } else {
    |  ____________________^
428 | |                 if let Some(size) = size {
429 | |                     buffer.extend_zeros(size);
430 | |                 } else {
431 | |                     prepend += 1;
432 | |                 }
433 | |             }
    | |_____________^
    |
    = note: `-D clippy::collapsible-if` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if
help: collapse nested if block
    |
427 |             } else if let Some(size) = size {
428 |                 buffer.extend_zeros(size);
429 |             } else {
430 |                 prepend += 1;
431 |             }
    |

Intrpduced in #9647 / 775a714 (though I had problems reproducing this locally)

[nevi-me]

@alamb I snuck in the fix here: ef64d00

Doesn't moving the 2 lines from the else change the logic?

[alamb]

Doesn't moving the 2 lines from the else change the logic?

Yes, I believe it does. 🤦 Thanks @nevi-me

[github-actions]

https://issues.apache.org/jira/browse/ARROW-12015