JSON writer - write null structs as null, instead of struct with nulls

[Jefffrey]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

See note here:

arrow-rs/arrow-json/src/writer.rs

Lines 1315 to 1328 in 31b5724

	// NOTE: The last value should technically be {"list": [null]} but it appears
	// that implementations differ on the treatment of a null struct.
	// It would be more accurate to return a null struct, so this can be done
	// as a follow up.
	assert_json_eq(
	&buf,
	r#"{"list":[{"ints":1}]}
	{"list":[{}]}
	{"list":[]}
	{}
	{"list":[{}]}
	{"list":[{}]}
	"#,
	);

Describe the solution you'd like

When writing a struct array, if there is a null then should write null instead of {}

Describe alternatives you've considered

Keep behaviour as is, writing {} for null entry in struct array. Remove above note in code and clearly document this is expected behaviour.

Additional context

[tustvold]

I think this is actually just a bug

[tustvold]

label_issue.py automatically added labels {'arrow'} from #5133

[tustvold]

label_issue.py automatically added labels {'object-store'} from #5065