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

airflow/api_connexion/schemas/dag_run_schema.py
"""Schema for DAGRun"""
    class Meta:
        """Meta"""
        model = DagRun
        dateformat = "iso"
    run_id = auto_field(data_key='dag_run_id')
    dag_id = auto_field(dump_only=True)
    execution_date = auto_field()
    start_date = auto_field(dump_only=True)
    end_date = auto_field(dump_only=True)
    state = DagStateField(dump_only=True)
    external_trigger = auto_field(default=True, dump_only=True)
    conf = ConfObject()
    @pre_load
    def autogenerate(self, data, **kwargs):
        """Auto generate run_id and execution_date if they are not loaded"""
        if "execution_date" not in data.keys():
            data["execution_date"] = str(timezone.utcnow())
        if "dag_run_id" not in data.keys():
            data["dag_run_id"] = DagRun.generate_run_id(
                DagRunType.MANUAL, timezone.parse(data["execution_date"])
            )
        return data
class DAGRunCollection(NamedTuple):