fix: stack nil pointer error

[cr7258]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When I run --watch to monitor resource change, I got this error:

kusion apply --watch
 ✔︎  Generating Intent in the Stack dev...                                                                                                                                                                                           
Stack: dev  ID                                                          Action
* ├─       v1:Namespace:wordpress                                      Create
* ├─       v1:Secret:wordpress:wordpress-db-local-secret               Create
* ├─       v1:Secret:wordpress:wordpress-db                            Create
* ├─       v1:Service:wordpress:wordpress-dev-wordpress-private        Create
* ├─       v1:Service:wordpress:wordpress-db-local-service             Create
* ├─       v1:PersistentVolumeClaim:wordpress:wordpress-db-local-pvc   Create
* ├─       apps/v1:Deployment:wordpress:wordpress-dev-wordpress        Create
* └─       apps/v1:Deployment:wordpress:wordpress-db-local-deployment  Create


? Do you want to apply these diffs? yes
Start applying diffs ...
 SUCCESS  Create v1:Namespace:wordpress success                                                                                                                                                                                      
 SUCCESS  Create v1:Secret:wordpress:wordpress-db-local-secret success                                                                                                                                                               
 SUCCESS  Create v1:Secret:wordpress:wordpress-db success                                                                                                                                                                            
 SUCCESS  Create v1:Service:wordpress:wordpress-db-local-service success                                                                                                                                                             
 SUCCESS  Create v1:Service:wordpress:wordpress-dev-wordpress-private success                                                                                                                                                        
 SUCCESS  Create v1:PersistentVolumeClaim:wordpress:wordpress-db-local-pvc success                                                                                                                                                   
 SUCCESS  Create apps/v1:Deployment:wordpress:wordpress-dev-wordpress success                                                                                                                                                        
 SUCCESS  Create apps/v1:Deployment:wordpress:wordpress-db-local-deployment success                                                                                                                                                  
Create apps/v1:Deployment:wordpress:wordpress-db-local-deployment success [8/8] ███████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████ 100% | 3s
Apply complete! Resources: 8 created, 0 updated, 0 deleted.

Start watching changes ...
 ❌  runtime error: invalid memory address or nil pointer dereference
exit status 1

The stack is null, resulting nil pointer error:
https://github.com/cr7258/kusion/blob/1dab8e16ae3189fa707e721f5967e68551ecf003/pkg/util/kube/config/config.go#L28

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Additional documentation e.g., design docs, usage docs, etc.:

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[cr7258]

I have read the CLA Document and I hereby sign the CLA

[coveralls]

Pull Request Test Coverage Report for Build 6971201106

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 14 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.1%) to 75.733%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/engine/operation/parser/parser.go	3	79.27%
pkg/engine/operation/parser/spec_parser.go	4	56.1%
pkg/cmd/apply/options.go	7	71.83%

Totals
Change from base Build 6965344887:	-0.1%
Covered Lines:	6978
Relevant Lines:	9214

---

💛 - Coveralls

[SparkYuan]

@cr7258 Thanks for your contributions. Welcome!

[healthjyk]

It seems an unexpected bug. Thanks for your contribution!