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

airflow/api_connexion/endpoints/dag_run_endpoint.py
#
#
from connexion import NoContent
from flask import current_app, g, request
from marshmallow import ValidationError
from airflow.api_connexion import security
from airflow.api_connexion.exceptions import AlreadyExists, BadRequest, NotFound
from airflow.api_connexion.parameters import check_limit, format_datetime, format_parameters
from airflow.api_connexion.schemas.dag_run_schema import (
    DAGRunCollection,
    dagrun_collection_schema,
    dagrun_schema,
    dagruns_batch_form_schema,
)
from airflow.models import DagModel, DagRun
from airflow.security import permissions
from airflow.utils.session import provide_session
from airflow.utils.types import DagRunType
@security.requires_access(
    [
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
        (permissions.ACTION_CAN_DELETE, permissions.RESOURCE_DAG_RUN),
    ]
)
@provide_session
def delete_dag_run(dag_id, dag_run_id, session):