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
@provide_session
    def test_should_respond_200(self, session):
        self._create_test_dag_run()
        result = session.query(DagRun).all()
        assert len(result) == 2
        response = self.client.get(
            "api/v1/dags/TEST_DAG_ID/dagRuns", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 200
        assert response.json == {
            "dag_runs": [
                {
                    'dag_id': 'TEST_DAG_ID',
                    'dag_run_id': 'TEST_DAG_RUN_ID_1',
                    'end_date': None,
                    'state': 'running',
                    'execution_date': self.default_time,
                    'external_trigger': True,
                    'start_date': self.default_time,
                    'conf': {},
                },
                {
                    'dag_id': 'TEST_DAG_ID',
                    'dag_run_id': 'TEST_DAG_RUN_ID_2',
                    'end_date': None,