fix: Use null-conditional operator to access FinishedAt

[HofmeisterAn]

What does this PR do?

This PR fixes a NRE that can occur when a container is created with the auto-remove option and then manually stopped. If Docker removes the container, the _container field is set to a default value that lacks a valid State property. The fix adds a null-conditional operator to safely access State.

While not perfect, the implementation already falls back to UtcNow (for the StoppedTime property), so the behavior is preserved. All other accesses to State were double-checked and are already NRE-safe.

Why is it important?

This change prevents users from encountering unexpected exceptions.

Related issues

• Closes [Bug]: Possible NullReferenceExcetion on StoppedTime calculation when DockerContainer.StopAsync is called #1496

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