Fix indefinite loop when when validators are present, but VC does not know indices yet. (#6615)

Fix block pruning when no validators seen.

beacon_chain/validator_client/duties_service.nim

@@ -114,7 +114,7 @@ proc pollForAttesterDuties*(service: DutiesServiceRef,
     vc = service.client
     indices = toSeq(vc.attachedValidators[].indices())
     relevantDuties =
-      block:
+      if len(indices) > 0:
         var duties: seq[RestAttesterDuty]
         block mainLoop:
           while true:
@@ -153,6 +153,8 @@ proc pollForAttesterDuties*(service: DutiesServiceRef,
                     duties.add(duty)
                 break mainLoop
         duties
+      else:
+        default(seq[RestAttesterDuty])
 
   template checkReorg(a, b: untyped): bool =
     not(a.dependentRoot == b.get())