[analysis plugin] fix nil pointer in executeStage

[t-kikuc]

What this PR does:

Why we need it:

e.targetDS caused a nil panic since it is not used at all.

Which issue(s) this PR fixes:

related to #6068

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):

[Warashi]

👍🏻

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 28.77%. Comparing base (637fc34) to head (50f4e17).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...p/pipedv1/plugin/analysis/executestage/analysis.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6203      +/-   ##
==========================================
- Coverage   28.77%   28.77%   -0.01%     
==========================================
  Files         559      559              
  Lines       59520    59520              
==========================================
- Hits        17126    17125       -1     
  Misses      41080    41080              
- Partials     1314     1315       +1     

Flag	Coverage Δ
.	23.24% <ø> (+<0.01%)	⬆️
.-pkg-app-pipedv1-plugin-analysis	32.64% <0.00%> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	60.24% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.48% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	39.49% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	55.73% <ø> (ø)
.-pkg-plugin-sdk	50.17% <ø> (-0.20%)	⬇️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.51% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[ffjlabo]

👍