[JavaScript] Structs don't append nulls properly

If you have a StructBuilder, then the `set` method (which is inherited from `Builder`) on it will modify the null-bitmap and then return directly due to this snippet:

public set(index: number, value: T['TValue'] | TNull) {
        if (this.setValid(index, this.isValid(value))) {
            this.setValue(index, value);
        }
        return this;
    }

 
I believe this breaks the spec, as it results in the children arrays not having their lengths and null-counts increased. (At least the Rust implementation expects child arrays to be the same length as their parent struct array, and the spec seems to imply that's a requirement)

I think there's an easy fix which would be to call `this.setValue` for `StructBuilder`s regardless of `this.isValid(value)`

Related to https://issues.apache.org/jira/browse/ARROW-15651