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
{"pool": "test_pool_3"},
                ],
                True,
                (
                    "/api/v1/dags/example_python_operator/dagRuns/"
                    "TEST_DAG_RUN_ID/taskInstances?pool=test_pool_1,test_pool_2"
                ),
                2,
            ),
            (
                "test pool filter ~",
                [
                    {"pool": "test_pool_1"},
                    {"pool": "test_pool_2"},
                    {"pool": "test_pool_3"},
                ],
                True,
                "/api/v1/dags/~/dagRuns/~/taskInstances?pool=test_pool_1,test_pool_2",
                2,
            ),
            (
                "test queue filter",
                [
                    {"queue": "test_queue_1"},
                    {"queue": "test_queue_2"},
                    {"queue": "test_queue_3"},
                ],
                True,
                (
                    "/api/v1/dags/example_python_operator/dagRuns/TEST_DAG_RUN_ID"