@Override
    public List<Relationship> getDefault(Shape shape) {
        return shape.getMixins().isEmpty()
                ? Collections.emptyList()
                : initializeRelationships(shape, 0);
    }
Finds all neighbors of a shape, returning them as a list of
{@link Relationship} objects.

<p>Each neighbor shape that is not in the provided model will
result in a relationship where the optional
{@link Relationship#getNeighborShape() neighbor shape} is empty.

@see NeighborProvider#of
smithy-model/src/main/java/software/amazon/smithy/model/neighbor/NeighborVisitor.java