[ECS/Fargate] [request]: Reset environmentFiles using containerOverrides

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**Tell us about your request**

Let's say that there is a task definition with the below configuration:
```
(...)
"environmentFiles": [
{
"value": "arn:aws:s3:::BUCKET_NAME/environment.env",
"type": "s3"
}
(...)
```
If [this doc](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/taskdef-envfiles.html) was correctly followed, the ECS task will start and the variable(s) will be available.

Let's also say that while calling the [RunTask](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_RunTask.html) API, the command provided is trying to reset the `environmentFiles` parameter.

**Which service(s) is this request for?**
Fargate, ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

I tried it with different approaches and none of them were able to remove the `environmentFiles` from the Task Definition. Please follow below the commands and the respective outcome:

```
$ aws ecs run-task --platform-version 1.4.0 --launch-type FARGATE --cluster Fargate --task-definition TASK_DEFINITION:VERSION_NUMBER --network-configuration 'awsvpcConfiguration={subnets=["SUBNET_ID"],securityGroups=["SG_ID"],assignPublicIp=ENABLED}' --overrides='{ "containerOverrides": [ { "name": "nginx-container", "environmentFiles": [ { "value": "", "type": "" } ] } ] }'

An error occurred (InvalidParameterException) when calling the RunTask operation: An unsupported environment file type was used. Specify a valid environment file type and try again.
```

```
$ aws ecs run-task --platform-version 1.4.0 --launch-type FARGATE --cluster Fargate --task-definition TASK_DEFINITION:VERSION_NUMBER --network-configuration 'awsvpcConfiguration={subnets=["SUBNET_ID"],securityGroups=["SG_ID"],assignPublicIp=ENABLED}' --overrides='{ "containerOverrides": [ { "name": "nginx-container", "environmentFiles": [ { } ] } ] }'

Parameter validation failed:
Missing required parameter in overrides.containerOverrides[0].environmentFiles[0]: "value"
Missing required parameter in overrides.containerOverrides[0].environmentFiles[0]: "type"
```

```
$ aws ecs run-task --platform-version 1.4.0 --launch-type FARGATE --cluster Fargate --task-definition TASK_DEFINITION:VERSION_NUMBER --network-configuration 'awsvpcConfiguration={subnets=["SUBNET_ID"],securityGroups=["SG_ID"],assignPublicIp=ENABLED}' --overrides='{ "containerOverrides": [ { "name": "nginx-container", "environmentFiles": [ ] } ] }'

Task runs with the previous configured `environmentFiles`. In other words, the `environmentFiles` is not `null`.
```

```
$ aws ecs run-task --platform-version 1.4.0 --launch-type FARGATE --cluster Fargate --task-definition TASK_DEFINITION:VERSION_NUMBER --network-configuration 'awsvpcConfiguration={subnets=["SUBNET_ID"],securityGroups=["SG_ID"],assignPublicIp=ENABLED}' --overrides='{ "containerOverrides": [ { "name": "nginx-container", "environmentFiles": null } ] }'

Parameter validation failed:
Invalid type for parameter overrides.containerOverrides[0].environmentFiles, value: None, type: , valid types: ,
```

**Are you currently working around this issue?**
Manually remove it from the Task Definition.

**Additional context**
Anything else we should know?

Let me know if it is supported and what would be the correct command in order to reset the `environmentFiles`.

Start with the ECS RunTask API documentation and the containerOverrides and environmentFiles parameters described in the issue. Compare the reported CLI outcomes with the documented override semantics; done would be a confirmed supported reset behavior or a clearly documented limitation and workaround.

Written by the indexing model from the issue text.