In `bp_activity_can_comment_reply()` check for and allow an empty `$comment` parameter, and set to `bp_activity_current_comment()` if so. This prevents debug notices and allows for cleaner comparisons in template parts.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8955 cdf35c40-ae34-48e0-9cc9-0c9da1808c22