Triggering a dagrun with naive datetime raises an HTML page instead of JSON

**Apache Airflow version**:2.0


**What happened**:

An HTML page was returned instead of json when the REST API was triggered with a naive datetime

**What you expected to happen**:

I expected it to raise error status 400 indicating that it's a bad request

**How to reproduce it**:
1. Start the web server and scheduler inside breeze.
2. Make a post request to this endpoint: http://localhost:28080/api/v1/dags/example_bash_operator/dagRuns
 to trigger dagruns using this request body: {"execution_date": "2020-11-09T16:25:56.939143"}. or any naive datetime
3. See that an HTML is raised instead of Json

tests/api_connexion/endpoints/test_dag_run_endpoint.py
)
        dag_runs.append(dagrun_model_1)
        dagrun_model_2 = DagRun(
            dag_id="TEST_DAG_ID",
            run_id="TEST_DAG_RUN_ID_2",
            run_type=DagRunType.MANUAL,
            execution_date=timezone.parse(self.default_time_2),
            start_date=timezone.parse(self.default_time),
            external_trigger=True,
        )
        dag_runs.append(dagrun_model_2)
        if extra_dag:
            for i in range(3, 5):
                dags.append(DagModel(dag_id='TEST_DAG_ID_' + str(i)))
                dag_runs.append(
                    DagRun(
                        dag_id='TEST_DAG_ID_' + str(i),
                        run_id='TEST_DAG_RUN_ID_' + str(i),
                        run_type=DagRunType.MANUAL,
                        execution_date=timezone.parse(self.default_time_2),
                        start_date=timezone.parse(self.default_time),
                        external_trigger=True,
                    )
                )
        if commit:
            with create_session() as session:
                session.add_all(dag_runs)
                session.add_all(dags)
        return dag_runs
class TestDeleteDagRun(TestDagRunEndpoint):