default <FK_ENTITY> Stream<ENTITY> findByNullable(
        HasNullableFinder<FK_ENTITY, ENTITY> fkField,
        FK_ENTITY fkEntity) {

        return finderByNullable(fkField).apply(fkEntity);
    }
Retrieves and returns a stream of Entities (with one or zero elements)
that matches the given a foreign key relation (foreign field and entity).
For example, if there is an entity Carrot with a FK to Hare using the
column "hare", then hares.find(Carrot.HARE, carrot) will return a stream
with the Hare that the Carrot.HARE field is pointing to or Stream.empty()
if the hare column in carrot was null. I.e. The returned Stream will
contain either zero or one element.

@param <FK_ENTITY> the type of the foreign entity

@param fkField the foreign key field
@param fkEntity the foreign key entity
@return an Entity (if any) that matches the given a foreign key relation
(foreign field and entity)
runtime-parent/runtime-core/src/main/java/com/speedment/runtime/core/manager/Manager.java