Attempted to assign id from null one-to-one property

catch very unexpected exception while updgrading hibernate version 5.2.12 -> 5.2.15
in scenario when
there are object whith null OneToOne property in database
and it is updated with object with non null OneToOne property.

i create test project to reproduce this case - https://github.com/evg345/spring-data-hibernate-bug

it seems that some versions of Hibernate are broken

• 5.2.12.Final = GOOD

• 5.2.13.Final = GOOD

• 5.2.14.Final = Exception

• 5.2.15.Final = Exception

• 5.2.16.Final = Exception

i create test to reproduce https://github.com/evg345/hibernate-test-case-templates/blob/master/orm/hibernate-orm-5/src/test/java/org/hibernate/bugs/HHH12436/OneToOnePropertyTestCase.java
and make pulll request also