**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
if dag_run and dag_run.external_trigger:
            prev_execution_date = self.execution_date
            next_execution_date = self.execution_date
        else:
            prev_execution_date = task.dag.previous_schedule(self.execution_date)
            next_execution_date = task.dag.following_schedule(self.execution_date)
        next_ds = None
        next_ds_nodash = None
        if next_execution_date:
            next_ds = next_execution_date.strftime('%Y-%m-%d')
            next_ds_nodash = next_ds.replace('-', '')
            next_execution_date = pendulum.instance(next_execution_date)
        prev_ds = None
        prev_ds_nodash = None
        if prev_execution_date:
            prev_ds = prev_execution_date.strftime('%Y-%m-%d')
            prev_ds_nodash = prev_ds.replace('-', '')
            prev_execution_date = pendulum.instance(prev_execution_date)
        ds_nodash = ds.replace('-', '')
        ts_nodash = self.execution_date.strftime('%Y%m%dT%H%M%S')
        ts_nodash_with_tz = ts.replace('-', '').replace(':', '')
        yesterday_ds_nodash = yesterday_ds.replace('-', '')
        tomorrow_ds_nodash = tomorrow_ds.replace('-', '')
        ti_key_str = f"{task.dag_id}__{task.task_id}__{ds_nodash}"
        if task.params:
            params.update(task.params)
        if conf.getboolean('core', 'dag_run_conf_overrides_params'):
            self.overwrite_params_with_dag_run_conf(params=params, dag_run=dag_run)
        class VariableAccessor: