fix: allow empty cacertfile pem

[zhongwencool]

Fixes https://emqx.atlassian.net/browse/EMQX-10760

Summary

🤖 Generated by Copilot at b733adc

Refactor and improve SSL options handling for emqx. Use atoms instead of strings for SSL options schema, allow empty cacertfile option, and enhance error handling and testing for SSL files.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update