[ECS/Fargate] [request]: Support environment variables with blank values in S3 environment files

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**Tell us about your request**
This request is similar to https://github.com/aws/amazon-ecs-cli/pull/167 for ECS/EC2, but for ECS/Fargate instead.

Currently for ECS/Fargate, if a container definition references an environment variable file stored in S3 that contains variables with blank values, those variables will not show up in the container. For example: let's say I create a file with variable `SOMETHING=`, and store it in S3. Next, I set my container definition to use this file inside my task definition, and create a task from the task definition. I'd expect`printenv` in the resulting task's container to display the variable `SOMETHING=`, but instead the variable isn't displayed at all.

I would like for containers to have these variables set regardless of their values.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We're using a Docker image that runs an application expecting certain environment variables exist, even if they have a blank value. If environment variables with blank values are not set on the container, the application will fail to start.

**Are you currently working around this issue?**
By directly setting environment variables with blank values in the container definition instead of referencing an environment variable file that contains variables with blank values.

**Additional context**
None

**Attachments**
None

The issue describes ECS/Fargate tasks using S3 environment files and the missing behavior for blank-valued variables; no repository files, tests, or implementation entry point are provided. Reproduce the difference between an S3 file containing SOMETHING= and a direct container definition, and consider the work done when the resulting container exposes SOMETHING=.

Written by the indexing model from the issue text.