phase optional lifecycle explain (#4325)

Author: l-trotta

specification/ilm/explain_lifecycle/types.ts

@@ -39,7 +39,7 @@ export class LifecycleExplainManaged {
   lifecycle_date?: DateTime
   lifecycle_date_millis?: EpochTime<UnitMillis>
   managed: true
-  phase: Name
+  phase?: Name
   phase_time?: DateTime
   phase_time_millis?: EpochTime<UnitMillis>
   policy?: Name