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/task_instance_schema.py
"""Schema for handling the request of clearing task instance of a Dag"""
    dry_run = fields.Boolean(default=True)
    start_date = fields.DateTime(missing=None)
    end_date = fields.DateTime(missing=None)
    only_failed = fields.Boolean(missing=True)
    only_running = fields.Boolean(missing=False)
    include_subdags = fields.Boolean(missing=False)
    include_parentdag = fields.Boolean(missing=False)
    reset_dag_runs = fields.Boolean(missing=False)
    @validates_schema
    def validate_form(self, data, **kwargs):
        """Validates clear task instance form"""
        if data["only_failed"] and data["only_running"]:
            raise ValidationError("only_failed and only_running both are set to True")
        if data["start_date"] and data["end_date"]:
            if data["start_date"] > data["end_date"]:
                raise ValidationError("end_date is sooner than start_date")
class SetTaskInstanceStateFormSchema(Schema):
    """Schema for handling the request of setting state of task instance of a DAG"""
    dry_run = fields.Boolean(default=True)
    task_id = fields.Str(required=True)
    execution_date = fields.DateTime(required=True)
    include_upstream = fields.Boolean(required=True)
    include_downstream = fields.Boolean(required=True)
    include_future = fields.Boolean(required=True)
    include_past = fields.Boolean(required=True)
    new_state = TaskInstanceStateField(required=True, validate=validate.OneOf([State.SUCCESS, State.FAILED]))
class TaskInstanceReferenceSchema(Schema):