StructBuilder doesn't handle nulls correctly for empty structs

[emcake]

Describe the bug
StructBuilder allows the creation of empty structs, but doesn't handle nulls correctly when calling finish() to get the array.
To Reproduce
reproducing test:

    #[test]
    fn empty_struct_nulls() {
        let mut builder = StructBuilder::new(Fields::default(), vec![]);

        builder.append_null();
        builder.append_null();
        builder.append_null();

        let array = builder.finish();

        assert_eq!(array.len(), 3);
        assert_eq!(array.null_count(), 3);
    }

Expected behavior
The test passes.

Additional context
I think this is because of this line in StructArray:

https://github.com/apache/arrow-rs/blob/master/arrow-array/src/array/struct_array.rs#L116

which uses unwrap_or_default() to get the length. A potential fix is the unwrap_or with the null array length but I don't know the internals well enough to know if that's a correct fix.

[tustvold]

The challenge is the null buffer is technically optional. What is your use-case for a StructArray containing no data?

[emcake]

In this case I was replicating a protobuf structure, where proto allows empty message definitions:

message Foo {

}

message Bar {
    Foo foo = 1;
}

and I was doing this in a reflection-like context where I was just examining descriptors and creating structs for them. In the end I caught this in my code and replaced it with a differently-typed column.

The challenge is the null buffer is technically optional. What is your use-case for a StructArray containing no data?

Understood. I think it'd also be sensible to either use this as a fallback with another unwrap_or_default layer, or just bin if you have a) no field columns and b) no null buffer. Alternately if empty structs are unsupported then not allowing you to construct a builder with no fields seems sensible.

[tustvold]

label_issue.py automatically added labels {'arrow'} from #4845