**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
"""
        iso = execution_date.isoformat()
        cmd = ["airflow", "tasks", "run", dag_id, task_id, iso]
        if mark_success:
            cmd.extend(["--mark-success"])
        if pickle_id:
            cmd.extend(["--pickle", str(pickle_id)])
        if job_id:
            cmd.extend(["--job-id", str(job_id)])
        if ignore_all_deps:
            cmd.extend(["--ignore-all-dependencies"])
        if ignore_task_deps:
            cmd.extend(["--ignore-dependencies"])
        if ignore_depends_on_past:
            cmd.extend(["--ignore-depends-on-past"])
        if ignore_ti_state:
            cmd.extend(["--force"])
        if local:
            cmd.extend(["--local"])
        if pool:
            cmd.extend(["--pool", pool])
        if raw:
            cmd.extend(["--raw"])
        if file_path:
            cmd.extend(["--subdir", file_path])
        if cfg_path:
            cmd.extend(["--cfg-path", cfg_path])
        return cmd
    @property
    def log_filepath(self):