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
@parameterized.expand(
        [
            (
                "clear start date filter",
                "example_python_operator",
                [
                    {"execution_date": DEFAULT_DATETIME_1, "state": State.FAILED},
                    {
                        "execution_date": DEFAULT_DATETIME_1 + dt.timedelta(days=1),
                        "state": State.FAILED,
                    },
                    {
                        "execution_date": DEFAULT_DATETIME_1 + dt.timedelta(days=2),
                        "state": State.FAILED,
                    },
                ],
                "example_python_operator",
                {
                    "dry_run": True,
                    "start_date": DEFAULT_DATETIME_STR_2,