Task assignment fails with Cannot insert NULL into (""PEOPLEASSIGNMENTS_POTOWNERS"."ENTITY_ID") error

Issue

The actors assignment for a task is defined as a combination of fixed string and a variable. During process execution, the var1 variable could be set to an empty String due to business logic. Depending on the order of the string/variable in the actors assignment, this could work or lead to a failure during process execution:

    This works:
        bpmsAdmin,#{var1}
    This fails:
        #{var1},bpmsAdmin

Example failure when using Oracle:

12:28:11,344 WARN  [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (http-10.213.29.102:8080-3) SQL Error: 1400, SQLState: 23000
12:28:11,344 ERROR [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (http-10.213.29.102:8080-3) ORA-01400: cannot insert NULL into ("PEOPLEASSIGNMENTS_POTOWNERS"."ENTITY_ID")

12:28:11,344 INFO  [org.hibernate.engine.jdbc.batch.internal.AbstractBatchImpl] (http-10.213.29.102:8080-3) HHH000010: On release of batch it still contained JDBC statements
12:28:11,344 WARN  [org.jbpm.services.task.persistence.TaskTransactionInterceptor] (http-10.213.29.102:8080-3) Could not commit session: javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement