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
"""List of DAGRuns with metadata"""
    dag_runs: List[DagRun]
    total_entries: int
class DAGRunCollectionSchema(Schema):
    """DAGRun Collection schema"""
    dag_runs = fields.List(fields.Nested(DAGRunSchema))
    total_entries = fields.Int()
class DagRunsBatchFormSchema(Schema):
    """Schema to validate and deserialize the Form(request payload) submitted to DagRun Batch endpoint"""
    class Meta:
        """Meta"""
        datetimeformat = 'iso'
        strict = True
    page_offset = fields.Int(missing=0, min=0)
    page_limit = fields.Int(missing=100, min=1)
    dag_ids = fields.List(fields.Str(), missing=None)
    execution_date_gte = fields.DateTime(missing=None)
    execution_date_lte = fields.DateTime(missing=None)
    start_date_gte = fields.DateTime(missing=None)
    start_date_lte = fields.DateTime(missing=None)
    end_date_gte = fields.DateTime(missing=None)
    end_date_lte = fields.DateTime(missing=None)
dagrun_schema = DAGRunSchema()
dagrun_collection_schema = DAGRunCollectionSchema()
dagruns_batch_form_schema = DagRunsBatchFormSchema()