fix(workflow-engine): Handle missing ActionTarget in combined-rules serializer

[sentry]

This PR addresses issue SENTRY-5QVV, where a ValueError: None is not a valid ActionTarget was occurring during the serialization of workflow engine actions.

The root cause was that some Action model records, particularly those created via certain migration paths or for specific action types (like PagerDuty/OpsGenie), did not have a target_type key in their config JSON. When the WorkflowEngineActionSerializer attempted to convert this None value to an ActionTarget enum, it resulted in a ValueError.

The fix involves:

1. Adding a null-check in src/sentry/incidents/endpoints/serializers/workflow_engine_action.py to ensure that ActionTarget(target_type) is only called when target_type is not None. If target_type is None, the serialized targetType will also be None.
2. Adding a logger.warning when target_type is None to provide visibility into these cases, including the action_id and action_type for debugging.

Legal Boilerplate

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Fixes SENTRY-5QVV

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

[sentry]

Bug: Calling str(target_identifier) when target_identifier is None leads to a ValueError when int() is called on the resulting string "None" for certain action types.
_{Severity: HIGH}

Suggested Fix

Conditionally handle the case where target_identifier is None. Before converting it to a string, check if it is None and pass None or a sensible default to get_identifier_from_action. The get_identifier_from_action function should also be updated to handle a None or default value gracefully instead of assuming a valid integer string.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry/incidents/endpoints/serializers/workflow_engine_action.py#L79-L82

Potential issue: For PagerDuty or Sentry App actions, if the `target_identifier` is
missing from the action's configuration, it defaults to `None`. The code then
unconditionally converts this value to a string, resulting in `"None"`. This string is
passed to the `get_identifier_from_action` function, which attempts to cast it to an
integer using `int("None")`. This operation fails and raises a `ValueError`, causing a
server error during the serialization of the action.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[alexsohn1126]

@sentry add logging for when this happens

[alexsohn1126]

@sentry actually nvm just remove the tests

[alexsohn1126]

@sentry can you remove logging

[alexsohn1126]

@sentry actually no add logging

[alexsohn1126]

@sentry actually remove logging again