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(
        [
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?start_date_gte=2020-06-18T18:00:00+00:00",
                ["TEST_START_EXEC_DAY_18", "TEST_START_EXEC_DAY_19"],
            ),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?start_date_lte=2020-06-11T18:00:00+00:00",
                ["TEST_START_EXEC_DAY_10", "TEST_START_EXEC_DAY_11"],
            ),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?start_date_lte= 2020-06-15T18:00:00+00:00"
                "&start_date_gte=2020-06-12T18:00:00Z",
                [
                    "TEST_START_EXEC_DAY_12",
                    "TEST_START_EXEC_DAY_13",
                    "TEST_START_EXEC_DAY_14",
                    "TEST_START_EXEC_DAY_15",
                ],
            ),
            (