fix: dont try to unmarshal into JSON if data is empty

[nianyush]

fix kairos-io/kairos#3708

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.14%. Comparing base (73a9795) to head (d74bb9a).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/agent/TUIcustomizationPage.go	0.00%	4 Missing ⚠️
internal/agent/install.go	0.00%	3 Missing ⚠️
internal/agent/reset.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
- Coverage   43.18%   43.14%   -0.05%     
==========================================
  Files          60       60              
  Lines        7718     7726       +8     
==========================================
  Hits         3333     3333              
- Misses       4059     4067       +8     
  Partials      326      326              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mudler]

Looking good, thanks!