Fix panic in ECS driftdetection when a taskdef in livestates does not exist

[t-kikuc]

What this PR does / why we need it:

I added a nil check.

TaskDefs in livestates sometimes do not exist.
e.g. When a service is created but the taskset does not exist yet.

cf. LiveTask is set here:

pipecd/pkg/app/piped/livestatestore/ecs/store.go

Lines 66 to 99 in 1477e76

	var primaryTaskDef *types.TaskDefinition
	for _, taskSet := range service.TaskSets {
	if *taskSet.Status == "PRIMARY" {
	primaryTaskDef, err = s.client.GetTaskDefinition(ctx, *taskSet.TaskDefinition)
	if err != nil {
	return fmt.Errorf("failed to fetch ECS task definition %s: %w", *taskSet.TaskDefinition, err)
	}
	}
	tasks, err := s.client.GetTaskSetTasks(ctx, taskSet)
	if err != nil {
	return fmt.Errorf("failed to fetch ECS tasks of task set %s: %w", *taskSet.TaskSetArn, err)
	}
	taskSetTasks[*taskSet.TaskSetArn] = tasks
	}
	// Use the application ID tag as the key.
	appId := ""
	for _, tag := range service.Tags {
	if *tag.Key == provider.LabelApplication {
	appId = *tag.Value
	break
	}
	}
	if appId == "" {
	// Skip a service which is not managed by PipeCD.
	continue
	}
	apps[appId] = app{
	ecsManifests: provider.ECSManifests{
	ServiceDefinition: service,
	TaskDefinition: primaryTaskDef,
	},

Which issue(s) this PR fixes:

N/A

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 23.94%. Comparing base (1689c63) to head (1477e76).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5240      +/-   ##
==========================================
+ Coverage   22.92%   23.94%   +1.01%     
==========================================
  Files         420      437      +17     
  Lines       45336    46997    +1661     
==========================================
+ Hits        10392    11252     +860     
- Misses      34147    34841     +694     
- Partials      797      904     +107     

Flag	Coverage Δ
	23.94% <100.00%> (+1.01%)	⬆️

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[khanhtc1202]

Good catch 👍

[Warashi]

👍🏻