fix(incidents): Handle None target_type in workflow engine action serializer

[sentry]

This PR fixes a `ValueError: None is not a valid ActionTarget` that occurs when serializing `Action` objects within the workflow engine. The error happens because `obj.config.get("target_type")` can return `None` if the `target_type` key is missing from an Action's config. This `None` value is then passed directly to the `ActionTarget` enum constructor, which raises a `ValueError`.

The fix involves:

1. Updating the type annotation for target_type to int | None to accurately reflect its possible value.
2. Adding a None guard before calling ActionTarget(target_type) in the serializer. If target_type is None, the targetType field in the serialized output will also be None, preventing the ValueError.

Legal Boilerplate

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Fixes SENTRY-5NQG

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}