LeanMapper\Reflection\Property::isNullable PHP Method

    public function isNullable()
    {
        return $this->isNullable;
    }

 /**
  * @param Property $property
  * @param Relationship\BelongsToOne $relationship micro-optimalization
  * @param Filtering|null $filtering
  * @return Entity
  * @throws InvalidValueException
  */
 private function getBelongsToOneValue(Property $property, Relationship\BelongsToOne $relationship, Filtering $filtering = null)
 {
     $targetTable = $relationship->getTargetTable();
     $rows = $this->row->referencing($targetTable, $relationship->getColumnReferencingSourceTable(), $filtering, $relationship->getStrategy());
     $count = count($rows);
     if ($count > 1) {
         throw new InvalidValueException('There cannot be more than one entity referencing to entity ' . get_called_class() . " in property '{$property->getName()}' with m:belongToOne relationship.");
     } elseif ($count === 0) {
         if (!$property->isNullable()) {
             $name = $property->getName();
             throw new InvalidValueException("Property '{$name}' cannot be null in entity " . get_called_class() . '.');
         }
         return null;
     } else {
         $row = reset($rows);
         $entityClass = $this->mapper->getEntityClass($targetTable, $row);
         $entity = $this->entityFactory->createEntity($entityClass, $row);
         $this->checkConsistency($property, $entityClass, $entity);
         $entity->makeAlive($this->entityFactory);
         return $entity;
     }
 }