Fix people dropdown filter without defined keys

[yurabakhtin]

What kind of change does this PR introduce? (check at least one)

• Bugfix

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch if no hotfix
• All tests are passing
• Changelog was modified

Other information:
#6961

[what-the-diff]

PR Summary

• Issue Resolution
  • Resolved issue Admin User Create - Show all Profile Fields #6951, improving the people dropdown filter functionality.
• Updated getSelectItems method in CheckboxList.php and Select.php
  • This update ensures key-value pairs under options are handled correctly.
• New validateOptions method in Select.php
  • A new method added for verifying that options match the correct format (Key=>Value).
• Bug Fix in PeopleFilterPicker.php
  • Fixed a bug that was not ignoring empty or null items, thereby improving reliability.
• Update in humhub.ui.picker.js
  • Improved the templateResult method to correctly handle cases where item ids could be empty.