fix: prevent undefined key name when creating Secret

[ruibaby]

What type of PR is this?

/area ui
/kind bug
/milestone 2.20.x

What this PR does / why we need it:

修复使用 Secret 输入框创建 Secret 时，stringData 的 key 可能为 undefined 的问题。

Which issue(s) this PR fixes:

See halo-sigs/plugin-alist#23 (comment) for more

Special notes for your reviewer:

Does this PR introduce a user-facing change?

修复使用 Secret 输入框创建 Secret 时，stringData 的 key 可能为 undefined 的问题。

[JohnNiang]

Hi @ruibaby , 先修复一下 CI 报错：https://github.com/halo-dev/halo/actions/runs/11569963413/job/32204752485?pr=6964。

[ruibaby]

Hi @ruibaby , 先修复一下 CI 报错：https://github.com/halo-dev/halo/actions/runs/11569963413/job/32204752485?pr=6964。

fixed

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.25%. Comparing base (eff73dc) to head (90331fc).
Report is 42 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6964      +/-   ##
============================================
+ Coverage     56.99%   57.25%   +0.25%     
- Complexity     3999     4006       +7     
============================================
  Files           714      712       -2     
  Lines         24110    24123      +13     
  Branches       1585     1588       +3     
============================================
+ Hits          13742    13811      +69     
+ Misses         9756     9699      -57     
- Partials        612      613       +1     

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[JohnNiang]

It works very well.

/lgtm

[f2c-ci-robot]

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