one last image ref type fix

Author: nicktrn

packages/core/src/v3/schemas/runEngine.ts

@@ -217,7 +217,7 @@ export const DequeueMessageCheckpoint = z.object({
   id: z.string(),
   type: CheckpointType,
   location: z.string(),
-  imageRef: z.string(),
+  imageRef: z.string().nullish(),
   reason: z.string().nullish(),
 });
 export type DequeueMessageCheckpoint = z.infer<typeof DequeueMessageCheckpoint>;