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 the task instance reference schema"""
    task_id = fields.Str()
    dag_run_id = fields.Str()
    dag_id = fields.Str()
    execution_date = fields.DateTime()
    def get_attribute(self, obj, attr, default):
        """Overwritten marshmallow function"""
        task_instance_attr = ['task_id', 'execution_date', 'dag_id']
        if attr in task_instance_attr:
            obj = obj[0]  
            return get_value(obj, attr, default)
        return obj[1]
class TaskInstanceReferenceCollection(NamedTuple):
    """List of objects with metadata about taskinstance and dag_run_id"""
    task_instances: List[Tuple[TaskInstance, str]]
class TaskInstanceReferenceCollectionSchema(Schema):
    """Collection schema for task reference"""
    task_instances = fields.List(fields.Nested(TaskInstanceReferenceSchema))
task_instance_schema = TaskInstanceSchema()
task_instance_collection_schema = TaskInstanceCollectionSchema()
task_instance_batch_form = TaskInstanceBatchFormSchema()
clear_task_instance_form = ClearTaskInstanceFormSchema()
set_task_instance_state_form = SetTaskInstanceStateFormSchema()
task_instance_reference_schema = TaskInstanceReferenceSchema()
task_instance_reference_collection_schema = TaskInstanceReferenceCollectionSchema()