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/task_instance_schema.py
#
#
from typing import List, NamedTuple, Optional, Tuple
from marshmallow import Schema, ValidationError, fields, validate, validates_schema
from marshmallow.utils import get_value
from airflow.api_connexion.schemas.enum_schemas import TaskInstanceStateField
from airflow.api_connexion.schemas.sla_miss_schema import SlaMissSchema
from airflow.models import SlaMiss, TaskInstance
from airflow.utils.state import State
class TaskInstanceSchema(Schema):