ECSOperator: fix KeyError on missing exitCode

[ssatia]

exitCode is not a required property on the container status response and is missing if the container exits during the early stages of initialization. This change would lead to the missing key being interpreted as an erroneous exit code after the container is stopped, which is the desired behavior.

closes: #18089

[boring-cyborg]

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[potiuk]

Woudl it be possible to add a unit test covering that?

[ssatia]

Woudl it be possible to add a unit test covering that?

Added a UT

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!