**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
'outlets': task.outlets,
            'params': params,
            'prev_ds': prev_ds,
            'prev_ds_nodash': prev_ds_nodash,
            'prev_execution_date': prev_execution_date,
            'prev_execution_date_success': lazy_object_proxy.Proxy(
                lambda: self.get_previous_execution_date(state=State.SUCCESS)
            ),
            'prev_start_date_success': lazy_object_proxy.Proxy(
                lambda: self.get_previous_start_date(state=State.SUCCESS)
            ),
            'run_id': run_id,
            'task': task,
            'task_instance': self,
            'task_instance_key_str': ti_key_str,
            'test_mode': self.test_mode,
            'ti': self,
            'tomorrow_ds': tomorrow_ds,
            'tomorrow_ds_nodash': tomorrow_ds_nodash,
            'ts': ts,
            'ts_nodash': ts_nodash,
            'ts_nodash_with_tz': ts_nodash_with_tz,
            'var': {
                'json': VariableJsonAccessor(),
                'value': VariableAccessor(),
            },
            'yesterday_ds': yesterday_ds,
            'yesterday_ds_nodash': yesterday_ds_nodash,
        }
    def get_rendered_template_fields(self):