Fix nil pointer dereference error on spec.plugins config on app config not set

[Warashi]

What this PR does:

as title

Why we need it:

for safe execution

Which issue(s) this PR fixes:

Does this PR introduce a user-facing change?: No

• 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 28.47%. Comparing base (fbb422d) to head (237cf01).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5922      +/-   ##
==========================================
+ Coverage   28.42%   28.47%   +0.05%     
==========================================
  Files         520      520              
  Lines       56381    56407      +26     
==========================================
+ Hits        16024    16060      +36     
+ Misses      39093    39081      -12     
- Partials     1264     1266       +2     

Flag	Coverage Δ
.	23.74% <ø> (ø)
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (+0.62%)	⬆️
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	49.39% <100.00%> (+0.05%)	⬆️
.-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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[khanhtc1202]

Nice catch 👍