[JavaScript] Structs incorrectly initialise null values

let list_field = new Field('n1', new FixedSizeList(2, Float64), true);
let struct_field = new Field('foo', new Struct([list_field]), true);

let builder = new Builder({
type:struct_field.type,
nullValues: [null, undefined],
});

builder.append(null);

console.log('Builder:' + JSON.stringify(builder));
console.log('numChildren: ' + builder.numChildren);
console.log('nullCount: ' + builder.nullCount);
console.log('length: ' + builder.length);

let vec1 = builder.toVector();
console.log('Vector from Builder:' + vec1);
console.log('numChildren: ' + vec1.numChildren);
console.log('nullCount: ' + vec1.nullCount);
console.log('length: ' + vec1.length);

let vec2 = makeVector({
data: [null],
type:struct_field.type,
nullable:true,
});

console.log('Vector from makeVector:' + JSON.stringify(vec2));
console.log('numChildren: ' + vec2.numChildren);
console.log('nullCount: ' + vec2.nullCount);
console.log('length: ' + vec2.length);

Builder: \{"length":1,"finished":false,"type":{"children":[{"name":"n1","type":{"listSize":2,"children":[null]},"nullable":true,"metadata":{}}]},"children":[],"nullValues":[null,null],"stride":1,"_nulls":\{"buffer": ... ,"stride":0.125,"BYTES_PER_ELEMENT":1,"length":1,"numValid":0}}
numChildren: 0
nullCount: 1
length: 1

Vector from Builder:[]
numChildren: 1
nullCount: 1
length: 1

Vector from makeVector:[]
numChildren: 1
nullCount: 0
length: 0