Deletion: Handle the case of pruned objects.

lib/pleroma/user.ex

lib/pleroma/web/activity_pub/builder.ex

...ma/web/activity_pub/object_validators/delete_validator.ex

lib/pleroma/web/activity_pub/transmogrifier.ex

lib/pleroma/web/common_api/common_api.ex

test/tasks/user_test.exs

test/web/activity_pub/side_effects_test.exs

.../web/activity_pub/transmogrifier/delete_handling_test.exs

test/web/common_api/common_api_test.exs