[SDK] Fix nil pointer plugin config

[ffjlabo]

What this PR does:

Allow users not to set plugin config in the piped config.

Why we need it:

Which issue(s) this PR fixes:

Part of #5530
Follow #6161

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 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.56%. Comparing base (04c196a) to head (143689d).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/sdk/plugin.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6164      +/-   ##
==========================================
- Coverage   28.59%   28.56%   -0.03%     
==========================================
  Files         535      535              
  Lines       57242    57288      +46     
==========================================
  Hits        16367    16367              
- Misses      39603    39649      +46     
  Partials     1272     1272              

Flag	Coverage Δ
.	23.13% <ø> (ø)
.-pkg-app-pipedv1-plugin-analysis	62.85% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	60.91% <ø> (-0.68%)	⬇️
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.48% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	38.31% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	68.62% <ø> (ø)
.-pkg-plugin-sdk	50.24% <0.00%> (-0.13%)	⬇️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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