**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

tests/jobs/test_scheduler_job.py
"""
        Test that scheduler._create_dag_runs does not raise an error when the DAG does not exist
        in serialized_dag table
        """
        dag = DAG(dag_id='test_scheduler_create_dag_runs_does_not_raise_error', start_date=DEFAULT_DATE)
        DummyOperator(
            task_id='dummy',
            dag=dag,
        )
        dagbag = DagBag(
            dag_folder=os.devnull,
            include_examples=False,
            read_dags_from_db=False,
        )
        dagbag.bag_dag(dag=dag, root_dag=dag)
        
        DAG.bulk_write_to_db(dagbag.dags.values())
        dag_model = DagModel.get_dagmodel(dag.dag_id)
        scheduler = SchedulerJob(subdir=os.devnull, executor=self.null_exec)
        scheduler.processor_agent = mock.MagicMock()
        with create_session() as session, self.assertLogs(
            'airflow.jobs.scheduler_job', level="ERROR"
        ) as log_output:
            scheduler._create_dag_runs([dag_model], session)
            assert (
                "airflow.exceptions.SerializedDagNotFound: DAG "
                "'test_scheduler_create_dag_runs_does_not_raise_error' not found in serialized_dag table"
            ) in log_output.output[0]
    def test_bulk_write_to_db_external_trigger_dont_skip_scheduled_run(self):