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Getting `Submission failed. KeyError: 'env'` when attempting a "Hello World" prefect flow on ECS

Using the <https://prefecthq.github.io/prefect-aws/|prefect-aws guide> and using this invocation to build a deployment:
```prefect deployment build run.py:main \
    -n jack-test-1 \
    -ib ecs-task/ecs-block \
    -sb s3/flow-storage-block \
    --pool some-pool \
    --override env.EXTRA_PIP_PACKAGES=prefect-aws```

Here is <https://prefect-community.slack.com/archives/CL09KU1K7/p1680768652711329|another thread> where the same error was present. They seemed to get around it by specifying the `--override` elsewhere. But removing the `--override` from the above invocation leads to boto3 permissions errors.

I suspect this is because env is set to `None` in your `ecs-block` block right now.

If you use the UI to set env to `{}`, and then try your deployment command again, does it work?

It looks like the default for `env` is supposed to be <https://github.com/PrefectHQ/prefect-aws/blob/ecd36fe699f504784828dfb1e6832306cde5f64b/prefect_aws/ecs.py#L423|an empty dict>, which would prevent this from happening.

But I just created a new ECS block and my env was None, which I believe will cause the problem you saw <https://github.com/PrefectHQ/prefect/blob/main/src/prefect/agent.py#L413|right about here> when it tries to look up `env` in the block document.