SLAMiss is nullable and not always given back when pulling task insta…

…nces

See apache/airflow-client-python#50 for an example of the related error

airflow/www/static/js/types/api-generated.ts

@@ -1156,7 +1156,7 @@ export interface components {
       timestamp?: string;
       description?: string | null;
       notification_sent?: boolean;
-    };
+    } | null;
     Trigger: {
       id?: number;
       classpath?: string;