<!--

Welcome to Apache Airflow!  For a smooth issue process, try to answer the following questions.
Don't worry if they're not all applicable; just try to include what you can :-)

If you need to include code snippets or logs, please put them in fenced code
blocks.  If they're super-long, please use the details tag like
<details><summary>super-long log</summary> lots of stuff </details>

Please delete these comment blocks before submitting the issue.

-->

<!--

IMPORTANT!!!

PLEASE CHECK "SIMILAR TO X EXISTING ISSUES" OPTION IF VISIBLE
NEXT TO "SUBMIT NEW ISSUE" BUTTON!!!

PLEASE CHECK IF THIS ISSUE HAS BEEN REPORTED PREVIOUSLY USING SEARCH!!!

Please complete the next sections or the issue will be closed.
These questions are the first thing we need to know to understand the context.

-->

**Apache Airflow version**:
2.0

**Environment**:

- **OS** ubuntu 18.04
- **Kernel** 5.4.0-47-generic

**What happened**:
When I use list task instances REST api, I can not get the instances that status is `no_status`. 

```
### Get Task Instances
POST  {{baseUrl}}/dags/~/dagRuns/~/taskInstances/list
Authorization: Basic admin:xxx
Content-Type: application/json

{
  "dag_ids": ["stop_dags"]
}

or 

{
    "dag_ids": ["stop_dags"],
    "state": ["null"]
}
```

**What you expected to happen**:
include all state task instances when I don't have specific states.

<!-- What do you think went wrong? -->

**How to reproduce it**:
use REST test tools like postman to visit the api.

**Anything else we need to know**:
I can not find the REST api that I get all the dag runs instances with specific state, maybe should extend the REST api.

Thanks!

airflow/api_connexion/schemas/task_instance_schema.py
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):