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/schemas/dag_run_schema.py
#
#
#
import json
from typing import List, NamedTuple
from marshmallow import fields, pre_load
from marshmallow.schema import Schema
from marshmallow_sqlalchemy import SQLAlchemySchema, auto_field
from airflow.api_connexion.schemas.enum_schemas import DagStateField
from airflow.models.dagrun import DagRun
from airflow.utils import timezone
from airflow.utils.types import DagRunType
class ConfObject(fields.Field):
    """The conf field"""
    def _serialize(self, value, attr, obj, **kwargs):
        if not value:
            return {}
        return json.loads(value) if isinstance(value, str) else value
    def _deserialize(self, value, attr, data, **kwargs):
        if isinstance(value, str):
            return json.loads(value)
        return value
class DAGRunSchema(SQLAlchemySchema):