**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
"""
        task = self.task
        self.test_mode = test_mode
        self.refresh_from_task(task, pool_override=pool)
        self.refresh_from_db(session=session)
        self.job_id = job_id
        self.hostname = get_hostname()
        actual_start_date = timezone.utcnow()
        Stats.incr(f'ti.start.{task.dag_id}.{task.task_id}')
        try:
            if not mark_success:
                context = self.get_template_context()
                self._prepare_and_execute_task_with_callbacks(context, task)
            self.refresh_from_db(lock_for_update=True)
            self.state = State.SUCCESS
        except AirflowSmartSensorException as e:
            self.log.info(e)
            return
        except AirflowSkipException as e:
            
            
            if e.args:
                self.log.info(e)
            self.refresh_from_db(lock_for_update=True)
            self.state = State.SKIPPED
            self.log.info(
                'Marking task as SKIPPED. '
                'dag_id=%s, task_id=%s, execution_date=%s, start_date=%s, end_date=%s',
                self.dag_id,
                self.task_id,