Table per subclass doesn't work with abstract base class in hierarchy

I wanted to create a hierarchy of classes with the **table per subclass** strategy (see example below). Since we have common functionality in our domain classes, we have an **abstract base class** that is extended by all other domain classes.

Note: if I remove the abstract base class from the hierarchy, the application starts and works as expected.

### Task List

- [x] Steps to reproduce provided
- [x] Stacktrace (if present) provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce
Put the following classes in the grails domain folder.
```
abstract class BaseEntity {
// common behavior for all entities in the application
}

class X extends BaseEntity {
static mapping = {
tablePerHierarchy false
}
}

class A extends X {
static hasMany = [bs: B]
}

class B extends X {
A a
}
```

### Expected Behaviour
I think the given class hierarchy is not that complex and pretty common and should just work.

### Actual Behaviour
An exception is thrown during application boot.
```
Caused by: org.hibernate.MappingException: Entities [example.A] and [example.B] are mapped with the same discriminator value 'null'.
at org.hibernate.persister.entity.SingleTableEntityPersister.addSubclassByDiscriminatorValue(SingleTableEntityPersister.java:450)
at org.hibernate.persister.entity.SingleTableEntityPersister.(SingleTableEntityPersister.java:424)
... 57 common frames omitted
```

### Environment Information

- **Operating System**: Windows 10
- **GORM Version:** 7.0.1
- **Grails Version (if using Grails):** Grails 3.3.10
- **JDK Version:** 8

### Example Application
The 4 given classes under "Steps to Reproduce" are sufficient to reproduce the exception in a newly created grails application.

Create a new Grails application and place the BaseEntity, X, A, and B classes from the reproduction in its domain folder. Start by running the application to confirm the Hibernate MappingException during boot, then inspect GORM table-per-subclass and abstract-base-class mapping behavior. Done means the hierarchy boots without duplicate null discriminator values.

Written by the indexing model from the issue text.