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
@parameterized.expand(
        [
            (
                "All fields present",
                {
                    "dag_run_id": "TEST_DAG_RUN",
                    "execution_date": "2020-06-11T18:00:00+00:00",
                },
            ),
            ("dag_run_id missing", {"execution_date": "2020-06-11T18:00:00+00:00"}),
            ("dag_run_id and execution_date missing", {}),
        ]
    )
    @provide_session
    def test_should_respond_200(self, name, request_json, session):