**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
'dag_id=%s, task_id=%s, execution_date=%s, start_date=%s, end_date=%s',
            self.dag_id,
            self.task_id,
            self._date_or_empty('execution_date'),
            self._date_or_empty('start_date'),
            self._date_or_empty('end_date'),
        )
        self.set_duration()
        if not test_mode:
            session.add(Log(self.state, self))
            session.merge(self)
        session.commit()
        if not test_mode:
            self._run_mini_scheduler_on_child_tasks(session)
    @provide_session
    @Sentry.enrich_errors
    def _run_mini_scheduler_on_child_tasks(self, session=None) -> None:
        if conf.getboolean('scheduler', 'schedule_after_task_execution', fallback=True):
            from airflow.models.dagrun import DagRun  
            try:
                
                dag_run = with_row_locks(
                    session.query(DagRun).filter_by(
                        dag_id=self.dag_id,
                        execution_date=self.execution_date,
                    ),
                    session=session,
                ).one()