Issue webhook doesn't fire when assignee is removed

[srir]

Environment

SaaS (https://sentry.io/)

Steps to Reproduce

1. Create an integration with issue webhooks enabled
2. Assign a user to an issue in Sentry
3. Observe webhook fires with action assigned
4. Remove the assignee from the same issue
5. Check webhook logs

Expected Result

Webhook fires with action assigned (with assignedTo: null) or a new unassigned action, allowing integrations to track when issues become unassigned.

Actual Result

No webhook is sent when an issue is unassigned. Integrations cannot detect unassignment without polling.

Product Area

Settings - Integrations

[linear]

ENG-6232

[getsantry]

Assigning to @getsentry/support for routing ⏲️

[getsantry]

Routing to @getsentry/product-owners-settings-integrations for triage ⏲️