fix: Don't show read marker without message afterwards

[SystemKeeper]

How to reproduce (all on ios)

• Create a thread
• Write a few messages outside of the thread
• Write a few messages inside of the thread
• Mark the conversation as unread with swiping the conversation for unread
• Enter the conversation
• See that the read marker is at the very bottom of the chat, without any message afterwards

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[SystemKeeper]

As discussed will either hide it (like web) or show a change the message to indicate that the message is in a thread