**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
:param mark_success: Whether to mark the task as successful
        :type mark_success: bool
        :param ignore_all_deps: Ignore all ignorable dependencies.
            Overrides the other ignore_* parameters.
        :type ignore_all_deps: bool
        :param ignore_depends_on_past: Ignore depends_on_past parameter of DAGs
            (e.g. for Backfills)
        :type ignore_depends_on_past: bool
        :param ignore_task_deps: Ignore task-specific dependencies such as depends_on_past
            and trigger rule
        :type ignore_task_deps: bool
        :param ignore_ti_state: Ignore the task instance's previous failure/success
        :type ignore_ti_state: bool
        :param local: Whether to run the task locally
        :type local: bool
        :param pickle_id: If the DAG was serialized to the DB, the ID
            associated with the pickled DAG
        :type pickle_id: Optional[int]
        :param file_path: path to the file containing the DAG definition
        :type file_path: Optional[str]
        :param raw: raw mode (needs more details)
        :type raw: Optional[bool]
        :param job_id: job ID (needs more details)
        :type job_id: Optional[int]
        :param pool: the Airflow pool that the task should run in
        :type pool: Optional[str]
        :param cfg_path: the Path to the configuration file
        :type cfg_path: Optional[str]
        :return: shell command that can be used to run the task instance
        :rtype: list[str]