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
"""Task instance schema"""
    task_id = fields.Str()
    dag_id = fields.Str()
    execution_date = fields.DateTime()
    start_date = fields.DateTime()
    end_date = fields.DateTime()
    duration = fields.Float()
    state = TaskInstanceStateField()
    _try_number = fields.Int(data_key="try_number")
    max_tries = fields.Int()
    hostname = fields.Str()
    unixname = fields.Str()
    pool = fields.Str()
    pool_slots = fields.Int()
    queue = fields.Str()
    priority_weight = fields.Int()
    operator = fields.Str()
    queued_dttm = fields.DateTime(data_key="queued_when")
    pid = fields.Int()
    executor_config = fields.Str()
    sla_miss = fields.Nested(SlaMissSchema, default=None)
    def get_attribute(self, obj, attr, default):
        if attr == "sla_miss":
            
            
            
            slamiss_instance = {"sla_miss": obj[1]}
            return get_value(slamiss_instance, attr, default)
        return get_value(obj[0], attr, default)
class TaskInstanceCollection(NamedTuple):