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.create_task_instances(session)
        self.create_task_instances(session, dag_id="example_skip_dag")
        response = self.client.get(
            "/api/v1/dags/example_python_operator/dagRuns/~/taskInstances",
            environ_overrides={"REMOTE_USER": "test"},
        )
        self.assertEqual(response.status_code, 200)
        count = session.query(TaskInstance).filter(TaskInstance.dag_id == "example_python_operator").count()
        self.assertEqual(count, response.json["total_entries"])
        self.assertEqual(count, len(response.json["task_instances"]))
    def test_should_raises_401_unauthenticated(self):
        response = self.client.get(
            "/api/v1/dags/example_python_operator/dagRuns/~/taskInstances",
        )
        assert_401(response)
    def test_should_raise_403_forbidden(self):
        response = self.client.get(
            "/api/v1/dags/example_python_operator/dagRuns/~/taskInstances",
            environ_overrides={'REMOTE_USER': "test_no_permissions"},
        )
        assert response.status_code == 403
class TestGetTaskInstancesBatch(TestTaskInstanceEndpoint):