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
self,
        session,
        dag_id: str = "example_python_operator",
        update_extras: bool = True,
        single_dag_run: bool = True,
        task_instances=None,
        dag_run_state=State.RUNNING,
    ):
        """Method to create task instances using kwargs and default arguments"""
        dag = self.dagbag.get_dag(dag_id)
        tasks = dag.tasks
        counter = len(tasks)
        if task_instances is not None:
            counter = min(len(task_instances), counter)
        for i in range(counter):
            if task_instances is None:
                pass
            elif update_extras:
                self.ti_extras.update(task_instances[i])
            else: