fix: curio: fix incorrect null check for varchar column (#11881)

* err check empty string

* log skipped tasks

lib/harmony/harmonytask/harmonytask.go

@@ -313,7 +313,8 @@ func (e *TaskEngine) pollerTryAllWork() bool {
 		resources.CleanupMachines(e.ctx, e.db)
 	}
 	for _, v := range e.handlers {
-		if v.AssertMachineHasCapacity() != nil {
+		if err := v.AssertMachineHasCapacity(); err != nil {
+			log.Debugf("skipped scheduling %s type tasks on due to %s", v.Name, err.Error())
 			continue
 		}
 		var unownedTasks []TaskID

storage/paths/db_index.go

@@ -723,7 +723,7 @@ func (dbi *DBIndex) StorageBestAlloc(ctx context.Context, allocate storiface.Sec
 						 FROM storage_path 
 						 WHERE available >= $1
 						 and NOW()-($2 * INTERVAL '1 second') < last_heartbeat
-						 and heartbeat_err is null
+						 and heartbeat_err = ''
 						 and (($3 and can_seal = TRUE) or ($4 and can_store = TRUE))
 						order by (available::numeric * weight) desc`,
 		spaceReq,