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_task_instance_endpoint.py
f"?start_date_gte={DEFAULT_DATETIME_STR_1}&start_date_lte={DEFAULT_DATETIME_STR_2}"
                ),
                2,
            ),
            (
                "test start date filter with ~",
                [
                    {"start_date": DEFAULT_DATETIME_1},
                    {"start_date": DEFAULT_DATETIME_1 + dt.timedelta(days=1)},
                    {"start_date": DEFAULT_DATETIME_1 + dt.timedelta(days=2)},
                ],
                True,
                (
                    "/api/v1/dags/~/dagRuns/~/taskInstances?start_date_gte"
                    f"={DEFAULT_DATETIME_STR_1}&start_date_lte={DEFAULT_DATETIME_STR_2}"
                ),
                2,
            ),
            (
                "test end date filter",
                [
                    {"end_date": DEFAULT_DATETIME_1},
                    {"end_date": DEFAULT_DATETIME_1 + dt.timedelta(days=1)},
                    {"end_date": DEFAULT_DATETIME_1 + dt.timedelta(days=2)},
                ],
                True,
                (
                    "/api/v1/dags/example_python_operator/dagRuns/TEST_DAG_RUN_ID/taskInstances?"
                    f"end_date_gte={DEFAULT_DATETIME_STR_1}&end_date_lte={DEFAULT_DATETIME_STR_2}"
                ),