**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
self._date_or_empty('execution_date'),
                self._date_or_empty('start_date'),
                self._date_or_empty('end_date'),
            )
        except AirflowRescheduleException as reschedule_exception:
            self.refresh_from_db()
            self._handle_reschedule(actual_start_date, reschedule_exception, test_mode)
            return
        except AirflowFailException as e:
            self.refresh_from_db()
            self.handle_failure(e, test_mode, force_fail=True, error_file=error_file)
            raise
        except AirflowException as e:
            self.refresh_from_db()
            
            
            if self.state in {State.SUCCESS, State.FAILED}:
                return
            else:
                self.handle_failure(e, test_mode, error_file=error_file)
                raise
        except (Exception, KeyboardInterrupt) as e:
            self.handle_failure(e, test_mode, error_file=error_file)
            raise
        finally:
            Stats.incr(f'ti.finish.{task.dag_id}.{task.task_id}.{self.state}')
        
        self.end_date = timezone.utcnow()
        self.log.info(
            'Marking task as SUCCESS. '