FIX: Topic timeline not updating in megatopics. (PR #14645)

Before this fix, jumping to posts using the topic timeline scrollbar will not update the counts since the topic scrollarea is not rerendered.

Follow-up to db337b10ee2af66f464082ca24bb33536da5fbde

GitHub

Megatopics do not have a stream so the streamLength will always be zero.

Thank you @davidtaylorhq