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
session,
    dag_id,
    start_date_gte=None,
    start_date_lte=None,
    execution_date_gte=None,
    execution_date_lte=None,
    end_date_gte=None,
    end_date_lte=None,
    offset=None,
    limit=None,
):
    """Get all DAG Runs."""
    query = session.query(DagRun)
    
    if dag_id == "~":
        appbuilder = current_app.appbuilder
        query = query.filter(DagRun.dag_id.in_(appbuilder.sm.get_readable_dag_ids(g.user)))
    else:
        query = query.filter(DagRun.dag_id == dag_id)
    dag_run, total_entries = _fetch_dag_runs(
        query,
        end_date_gte,
        end_date_lte,
        execution_date_gte,
        execution_date_lte,
        start_date_gte,
        start_date_lte,