DOC-3526: Chat can now update an empty document

[kemister85]

Ticket: DOC-3526 (TINY-14393) TINYMCE-14393

Site: Staging

Changes:

• Add an 8.7.0 TinyMCE AI bug fix release note: the editor now sends an empty paragraph to the model when the content is empty, so Chat prompts can apply changes directly to an empty document.

Pre-checks:

• Branch is correctly prefixed: feature/8.7.0/DOC-3526_TINY-14393
• Files have been included where required (if applicable).
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.