When posting the very first activity update, avoid checking for previous activities

Since r8251, Activity HeartBeat feature needs to "parse" the different classes of the first activity item displayed in the stream. But before doing so, we actually need to check there is at least one activity displayed.

props pross

Fixes #5528

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8264 cdf35c40-ae34-48e0-9cc9-0c9da1808c22