**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
partial_dag = self.task.dag.partial_subset(
                    self.task.downstream_task_ids,
                    include_downstream=False,
                    include_upstream=False,
                    include_direct_upstream=True,
                )
                dag_run.dag = partial_dag
                info = dag_run.task_instance_scheduling_decisions(session)
                skippable_task_ids = {
                    task_id
                    for task_id in partial_dag.task_ids
                    if task_id not in self.task.downstream_task_ids
                }
                schedulable_tis = [ti for ti in info.schedulable_tis if ti.task_id not in skippable_task_ids]
                for schedulable_ti in schedulable_tis:
                    if not hasattr(schedulable_ti, "task"):
                        schedulable_ti.task = self.task.dag.get_task(schedulable_ti.task_id)
                num = dag_run.schedule_tis(schedulable_tis)
                self.log.info("%d downstream tasks scheduled from follow-on schedule check", num)
                session.commit()
            except OperationalError as e:
                
                self.log.info(
                    f"Skipping mini scheduling run due to exception: {e.statement}",
                    exc_info=True,
                )
                session.rollback()
    def _prepare_and_execute_task_with_callbacks(self, context, task):