@Override
    public int compare(ENTITY first, ENTITY second) {
        final V f = get(first);
        final V s = get(second);
        if (f == null && s == null) return 0;
        else if (f == null) return 1;
        else if (s == null) return -1;
        else return f.compareTo(s);
    }
@param <ENTITY>     the entity type
@param <D>          the database type
@param <V>          the field type
@param <FK_ENTITY>  the foreign entity type

@author  Emil Forslund
@author  Per Minborg

@since  2.2.0
runtime-parent/runtime-field/src/main/java/com/speedment/runtime/field/internal/ComparableForeignKeyFieldImpl.java