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_dag_run_endpoint.py
@parameterized.expand(
        [
            ("api/v1/dags/TEST_DAG_ID/dagRuns?limit=1", ["TEST_DAG_RUN_ID1"]),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?limit=2",
                ["TEST_DAG_RUN_ID1", "TEST_DAG_RUN_ID2"],
            ),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?offset=5",
                [
                    "TEST_DAG_RUN_ID6",
                    "TEST_DAG_RUN_ID7",
                    "TEST_DAG_RUN_ID8",
                    "TEST_DAG_RUN_ID9",
                    "TEST_DAG_RUN_ID10",
                ],
            ),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?offset=0",
                [
                    "TEST_DAG_RUN_ID1",
                    "TEST_DAG_RUN_ID2",
                    "TEST_DAG_RUN_ID3",
                    "TEST_DAG_RUN_ID4",
                    "TEST_DAG_RUN_ID5",
                    "TEST_DAG_RUN_ID6",
                    "TEST_DAG_RUN_ID7",