fix(slack): Guard against NoneType selected_option in action webhooks

[sentry]

This PR fixes a TypeError: 'NoneType' object is not subscriptable occurring in src/sentry/integrations/slack/webhooks/action.py within the get_action_list method.

The root cause was that for Slack static_select and external_select action types, the code unconditionally attempted to subscript action_data['selected_option'] and its nested keys. However, Slack can send payloads where selected_option is None or missing (e.g., when a user opens or clears a select menu without making a selection), leading to the TypeError.

The fix involves:

• Safely retrieving selected_option using .get() and verifying it is a dictionary.
• If selected_option is None or not a dictionary, the specific action is now skipped, and an informational log (slack.action.missing_selected_option) is emitted to aid future debugging without crashing.
• All subsequent accesses to selected_option's nested keys (like text.text and value) now use .get() to prevent similar errors if those sub-keys are also missing.

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Fixes SENTRY-3ZYV