Improve audit events streaming header UX

What does this MR do and why?

Relates to #415350 (closed)

Improve audit events streaming header UX.

• Do not add empty header on mount
• Do not add empty header when only remaining header is deleted
• Show "No header created yet." message conditionally
• Update and improve specs
• Improve layout of table

Screenshots or screen recordings

	Before	After
on mount when no header present — first empty row could not be deleted before
after clicking "Add header" first time — first row still could not be deleted in before

note: the cell width of delete action is improved

How to set up and validate locally

Make sure you have an EE license

1. Go to any group
2. Go to "Secure" > "Audit events"
3. Click "Streams" tab
4. Create a new streaming destination by clicking "Add streaming destination"
5. Provide a "Destination URL"
6. Validate that now, with no header yet added, there is not any blank header added yet to the form
7. Validate that it shows "No header created yet." message
8. Validate that when clicking "Add header" it adds an empty header
9. Validate that this header is better aligned (width of cell for delete button is narrower than before and sized better)
10. Validate that this header can be deleted by clicking the trash icon and afterwards a blank header is not automatically added again
11. Validate that when adding 20 headers, the max headers message still appears

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.