Using default priority weight if not specified (#7954)

## What changed?
- Using default priority weight if not specified

## Why?
- In case new priority levels got introduced and break existing things

## How did you test it?
- [ ] built
- [ ] run locally and tested manually
- [x] covered by existing tests
- [ ] added new unit test(s)
- [ ] added new functional test(s)

service/history/configs/task.go

@@ -7,6 +7,8 @@ import (
 )
 
 var (
+	DefaultPriorityWeight = 1
+
 	DefaultActiveTaskPriorityWeight = map[tasks.Priority]int{
 		tasks.PriorityHigh:        10,
 		tasks.PriorityLow:         9,

service/history/queues/scheduler.go

@@ -106,10 +106,19 @@ func NewScheduler(
 			)
 		}
 
-		return configs.ConvertDynamicConfigValueToWeights(
+		weight, ok := configs.ConvertDynamicConfigValueToWeights(
 			namespaceWeights(namespaceName.String()),
 			logger,
 		)[key.Priority]
+		if !ok || weight <= 0 {
+			logger.Warn("Task priority weight not specified or is invalid, using default weight",
+				tag.TaskPriority(key.Priority.String()),
+				tag.NewInt("priority-weight", weight),
+				tag.NewInt("default-weight", configs.DefaultPriorityWeight),
+			)
+			weight = configs.DefaultPriorityWeight
+		}
+		return weight
 	}
 	channelWeightUpdateCh := make(chan struct{}, 1)
 	fifoSchedulerOptions := &tasks.FIFOSchedulerOptions{