Add a placeholder to the cards for the security-hub

[AllForNothing]

1. Fixes task 3 of Update style for the UI #19249

Thank you for contributing to Harbor!

Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

Merging #19536 (e622464) into main (7718134) will increase coverage by 0.07%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19536      +/-   ##
==========================================
+ Coverage   45.25%   45.32%   +0.07%     
==========================================
  Files         244      244              
  Lines       13320    13323       +3     
  Branches     2720     2720              
==========================================
+ Hits         6028     6039      +11     
+ Misses       6990     6981       -9     
- Partials      302      303       +1     

Flag	Coverage Δ
unittests	45.32% <100.00%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ability-summary/vulnerability-summary.component.ts	69.23% <100.00%> (+2.56%)	⬆️

... and 4 files with indirect coverage changes