Unable to insert using ORM: Instance <...> has a NULL identity key

Example:

```python
class Base(DeclarativeBase):
pass

class User(Base):
__tablename__ = "users"
__table_args__ = {"schema": "app"}

# Not a real column - just here to satisfy SQLAlchemy's requirement for a primary key for every table
id: Mapped[int] = orm.mapped_column(
BigInteger, primary_key=True, autoincrement=True
)

email: Mapped[Optional[str]] = orm.mapped_column(String)
```

The following code fails:

```python
engine = create_engine("bigquery://", echo=True)
Base.metadata.create_all(engine)

row = User(email="my@email.com")

with Session(engine) as session:
with session.begin():
session.add(row)
```

```
FlushError: Instance has a NULL identity key. If this is an auto-generated value, check that the database table allows generation of new primary key values, and that the mapped Column object is configured to expect these generated values. Ensure also that this flush() is not occurring at an inappropriate time, such as within a load() event.
```