FIX: Use created_at if last_posted_at is null (PR #13668)

Topic timeline showed Jan 1970 as last posted time if all posts of a topic were deleted or whispers.

GitHub

    assert.equal(query(".now-date").innerText, "Jul 8");

De-jQueryfying tests :wink:

(import query is needed then too, and .trim() might be required too)