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.ti_init.update(task_instances[i])
            ti = TaskInstance(task=tasks[i], **self.ti_init)
            for key, value in self.ti_extras.items():
                setattr(ti, key, value)
            session.add(ti)
            if single_dag_run is False:
                dr = DagRun(
                    dag_id=dag_id,
                    run_id=f"TEST_DAG_RUN_ID_{i}",
                    execution_date=self.ti_init["execution_date"],
                    run_type=DagRunType.MANUAL.value,
                    state=dag_run_state,
                )
                session.add(dr)
        if single_dag_run:
            dr = DagRun(
                dag_id=dag_id,
                run_id="TEST_DAG_RUN_ID",
                execution_date=self.default_time,
                run_type=DagRunType.MANUAL.value,
                state=dag_run_state,
            )
            session.add(dr)
        session.commit()
class TestGetTaskInstance(TestTaskInstanceEndpoint):
    @provide_session
    def test_should_respond_200(self, session):
        self.create_task_instances(session)
        response = self.client.get(
            "/api/v1/dags/example_python_operator/dagRuns/TEST_DAG_RUN_ID/taskInstances/print_the_context",