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
#
#
import datetime as dt
import getpass
import unittest
from unittest import mock
from parameterized import parameterized
from airflow.models import DagBag, DagRun, SlaMiss, TaskInstance
from airflow.security import permissions
from airflow.utils.session import provide_session
from airflow.utils.state import State
from airflow.utils.timezone import datetime
from airflow.utils.types import DagRunType
from airflow.www import app
from tests.test_utils.api_connexion_utils import assert_401, create_user, delete_user
from tests.test_utils.config import conf_vars
from tests.test_utils.db import clear_db_runs, clear_db_sla_miss
DEFAULT_DATETIME_1 = datetime(2020, 1, 1)
DEFAULT_DATETIME_STR_1 = "2020-01-01T00:00:00+00:00"
DEFAULT_DATETIME_STR_2 = "2020-01-02T00:00:00+00:00"
class TestTaskInstanceEndpoint(unittest.TestCase):