EclipseLink doesn't generate SQL UPDATE correctly if @ReturnUpdate is used in InheritanceType.JOINED

[rfelcman]

SQL UPDATE command is not correctly generated if @ReturnUpdate is used in InheritanceType.JOINED. It started from 2.7.7. It is related with fix for #709 .
Test case to simulate it:
DB Tables:

CREATE TABLE JPA22_RETURNINSERTUPDATE_MASTER_JOINED  (
    ID     NUMBER(15) PRIMARY KEY ,
    TYPE   VARCHAR2(50) NOT NULL
  );

CREATE TABLE JPA22_RETURNINSERTUPDATE_DETAIL_JOINED  (
    ID                  NUMBER(15) PRIMARY KEY ,
    DETAIL_NR           NUMBER(15),
    DETAIL_NR_VIRTUAL   NUMBER(15) AS ( DETAIL_NR * 10 ) VIRTUAL
);

There are two entities:

...
@Entity
@Table(name = "JPA22_RETURNINSERTUPDATE_MASTER_JOINED")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.STRING)
@SequenceGenerator(name = "AbstractEntityObjectSEQ_STORE", sequenceName = "TEST_RETURNINSERT_MASTER_JOINED_ID_SEQ", allocationSize = 1)
public class ReturnInsertUpdateMasterJoined implements Serializable {
...

...
@Entity
@Table(name = "JPA22_RETURNINSERTUPDATE_DETAIL_JOINED")
@PrimaryKeyJoinColumn(name = "id")
@DiscriminatorValue("A")
public class ReturnInsertUpdateDetailJoined extends ReturnInsertUpdateMasterJoined {
...

Fields in ReturnInsertUpdateDetailJoined

@Column(name = "detail_nr")
Long detailNumber = null;

@Column(name = "detail_nr_virtual", nullable = false, unique = true, updatable = false)
@ReturnInsert(returnOnly = true)
@ReturnUpdate
Long detailNumberVirtual = null;

There are two possible errors:

1. For field update in JPA22_RETURNINSERTUPDATE_DETAIL_JOINED table only

...
ReturnInsertUpdateDetailJoined returnInsertUpdateDetail  = em.find(ReturnInsertUpdateDetailJoined.class, 1L);
returnInsertUpdateDetail.setDetailNumber(22L);
returnInsertUpdateDetailJoined = em.merge(returnInsertUpdateDetail);
em.getTransaction().commit();
...

NPE is thrown

[EL Warning]: 2021-02-12 13:13:59.819--UnitOfWork(923013679)--Thread(Thread[main,5,main])--java.lang.NullPointerException
	at org.eclipse.persistence.platform.database.OraclePlatform.buildCallWithReturning(OraclePlatform.java:275)
	at org.eclipse.persistence.internal.expressions.SQLModifyStatement.buildCall(SQLModifyStatement.java:67)
	at org.eclipse.persistence.descriptors.ClassDescriptor.buildCallFromStatement(ClassDescriptor.java:885)
	at org.eclipse.persistence.internal.queries.StatementQueryMechanism.setCallFromStatement(StatementQueryMechanism.java:379)
...

2. Update fields in JPA22_RETURNINSERTUPDATE_MASTER_JOINED, JPA22_RETURNINSERTUPDATE_DETAIL_JOINED tables

...
ReturnInsertUpdateDetailJoined returnInsertUpdateDetail  = em.find(ReturnInsertUpdateDetailJoined.class, 1L);
returnInsertDetail.setType("TYPE_B");
returnInsertUpdateDetail.setDetailNumber(22L);
returnInsertUpdateDetailJoined = em.merge(returnInsertUpdateDetail);
em.getTransaction().commit();
...

leads into

Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.7.8.v20201217-ecdf3c32c4): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: java.sql.SQLException: ORA-06550: line 1, column 99:
PL/SQL: ORA-00904: "DETAIL_NR_VIRTUAL": invalid identifier
ORA-06550: line 1, column 7:
PL/SQL: SQL Statement ignored

Error Code: 6550
Call: BEGIN UPDATE JPA22_RETURNINSERTUPDATE_MASTER_JOINED SET type = ? WHERE (ID = ?) RETURNING detail_nr_virtual INTO ?; END;
	bind => [TYPE_B, 1, => detail_nr_virtual]
Query: UpdateObjectQuery(org.eclipse.persistence.jpa.returninsert.model.ReturnInsertUpdateDetailJoined@3e58d65e)
	at org.eclipse.persistence.exceptions.DatabaseException.sqlException(DatabaseException.java:333)
	at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.executeDirectNoSelect(DatabaseAccessor.java:908)
	at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.executeNoSelect(DatabaseAccessor.java:970)
...