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
(
                {"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",
                ],
            ),
            (
                {"execution_date_lte": "2020-06-13T18:00:00+00:00"},
                [
                    "TEST_START_EXEC_DAY_10",
                    "TEST_START_EXEC_DAY_11",
                    "TEST_START_EXEC_DAY_12",
                    "TEST_START_EXEC_DAY_13",
                ],
            ),
            (
                {"execution_date_gte": "2020-06-16T18:00:00+00:00"},
                [
                    "TEST_START_EXEC_DAY_16",
                    "TEST_START_EXEC_DAY_17",
                    "TEST_START_EXEC_DAY_18",
                    "TEST_START_EXEC_DAY_19",
                ],
            ),
        ]
    )
    def test_date_filters_gte_and_lte(self, payload, expected_dag_run_ids):