Array writer output empty when no record is written

[gwik]

Describe the bug

Array writer output is empty when no record is written.
Either when no batch or only empty batches are written the output is empty.
It's ok for line delimiter format but not for array format.

To Reproduce

let mut writer = ArrayWriter::new(vec![] as Vec<u8>);
writer.finish().unwrap();

Expected behavior

The buffer should contain be a valid json array [], instead it is empty.

[alamb]

label_issue.py automatically added labels {'arrow'} from #6614