default <FK_ENTITY> Function<FK_ENTITY, Stream<ENTITY>> finderByNullable(
        HasNullableFinder<FK_ENTITY, ENTITY> fkField) {

        return fkField.nullableFinder(getTableIdentifier(), this::stream);
    }
Returns a Function that, when it is applied, will produce an equivalent
result as if {@link #findByNullable(HasNullableFinder, Object)} was
called.

@param <FK_ENTITY> the type of the foreign entity

@param fkField the foreign key field
@return a function that returns an Entity (if any) that matches the given
a foreign key relation (foreign field and entity)

@see #findByNullable(HasNullableFinder, Object)
runtime-parent/runtime-core/src/main/java/com/speedment/runtime/core/manager/Manager.java