**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
"""Fetch rendered template fields from DB"""
        from airflow.models.renderedtifields import RenderedTaskInstanceFields
        rendered_task_instance_fields = RenderedTaskInstanceFields.get_templated_fields(self)
        if rendered_task_instance_fields:
            for field_name, rendered_value in rendered_task_instance_fields.items():
                setattr(self.task, field_name, rendered_value)
        else:
            try:
                self.render_templates()
            except (TemplateAssertionError, UndefinedError) as e:
                raise AirflowException(
                    "Webserver does not have access to User-defined Macros or Filters "
                    "when Dag Serialization is enabled. Hence for the task that have not yet "
                    "started running, please use 'airflow tasks render' for debugging the "
                    "rendering of template_fields."
                ) from e
    def get_rendered_k8s_spec(self):
        """Fetch rendered template fields from DB"""
        from airflow.models.renderedtifields import RenderedTaskInstanceFields
        rendered_k8s_spec = RenderedTaskInstanceFields.get_k8s_pod_yaml(self)
        if not rendered_k8s_spec:
            try:
                rendered_k8s_spec = self.render_k8s_pod_yaml()
            except (TemplateAssertionError, UndefinedError) as e:
                raise AirflowException(f"Unable to render a k8s spec for this taskinstance: {e}") from e
        return rendered_k8s_spec
    def overwrite_params_with_dag_run_conf(self, params, dag_run):