**Apache Airflow version**:
2.0.0

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):
1.15
**Environment**:

- **Cloud provider or hardware configuration**: 
GKE
- **OS** (e.g. from /etc/os-release):
Ubuntu 18.04

**What happened**:
I just migrated from 1.10.14 to 2.0.0. When I turn on some random dags, the scheduler crashes with the following error:

```python
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/airflow/jobs/scheduler_job.py", line 1275, in _execute
    self._run_scheduler_loop()
  File "/usr/local/lib/python3.6/dist-packages/airflow/jobs/scheduler_job.py", line 1377, in _run_scheduler_loop
    num_queued_tis = self._do_scheduling(session)
  File "/usr/local/lib/python3.6/dist-packages/airflow/jobs/scheduler_job.py", line 1533, in _do_scheduling
    num_queued_tis = self._critical_section_execute_task_instances(session=session)
  File "/usr/local/lib/python3.6/dist-packages/airflow/jobs/scheduler_job.py", line 1132, in _critical_section_execute_task_instances
    queued_tis = self._executable_task_instances_to_queued(max_tis, session=session)
  File "/usr/local/lib/python3.6/dist-packages/airflow/utils/session.py", line 62, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/airflow/jobs/scheduler_job.py", line 1034, in _executable_task_instances_to_queued
    if task_instance.pool_slots > open_slots:
TypeError: '>' not supported between instances of 'NoneType' and 'int'
```
**What you expected to happen**:

I expected those dags would have their tasks scheduled without problems.

**How to reproduce it**:

Can't reproduce it yet. Still trying to figure out if this happens only with specific dags or not.

**Anything else we need to know**:

I couldn't find in which context `task_instance.pool_slots` could be None

airflow/models/taskinstance.py
unixname = Column(String(1000))
    job_id = Column(Integer)
    pool = Column(String(50), nullable=False)
    pool_slots = Column(Integer, default=1)
    queue = Column(String(256))
    priority_weight = Column(Integer)
    operator = Column(String(1000))
    queued_dttm = Column(UtcDateTime)
    queued_by_job_id = Column(Integer)
    pid = Column(Integer)
    executor_config = Column(PickleType(pickler=dill))
    external_executor_id = Column(String(ID_LEN, **COLLATION_ARGS))
    
    
    __table_args__ = (
        Index('ti_dag_state', dag_id, state),
        Index('ti_dag_date', dag_id, execution_date),
        Index('ti_state', state),
        Index('ti_state_lkp', dag_id, task_id, execution_date, state),
        Index('ti_pool', pool, state, priority_weight),
        Index('ti_job_id', job_id),
    )
    dag_model = relationship(
        "DagModel",
        primaryjoin="TaskInstance.dag_id == DagModel.dag_id",
        foreign_keys=dag_id,
        uselist=False,
        innerjoin=True,
    )
    def __init__(self, task, execution_date: datetime, state: Optional[str] = None):