**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
Stats.incr(f'operator_failures_{task.task_type}', 1, 1)
        Stats.incr('ti_failures')
        if not test_mode:
            session.add(Log(State.FAILED, self))
        
        session.add(TaskFail(task, self.execution_date, self.start_date, self.end_date))
        
        
        
        
        
        
        
        
        
        if force_fail or not self.is_eligible_to_retry():
            self.state = State.FAILED
            if force_fail:
                log_message = "Immediate failure requested. Marking task as FAILED."
            else:
                log_message = "Marking task as FAILED."
            email_for_state = task.email_on_failure
        else:
            self.state = State.UP_FOR_RETRY
            log_message = "Marking task as UP_FOR_RETRY."
            email_for_state = task.email_on_retry
        self.log.info(
            '%s dag_id=%s, task_id=%s, execution_date=%s, start_date=%s, end_date=%s',
            log_message,
            self.dag_id,