Feed links should be visible only when functions return string

[sippis]

Currently, activity stream feed links are always visible even the feed link functions would return something else than string. This is in both legacy and nouveau templates.

Use case: I've disabled all feeds with bp_activity_enable_feeds filter. That didn't remove the feed links, so went ahead and used link filters like bp_get_sitewide_activity_feed_link and bp_get_activities_member_rss_link to return null. Now there's no real link but the element is still visible.

[imath]

The patch attached to #8454 will also fix this issue. So I'm going to close this one. Don't hesitate to reopen it if I understood wrong 👌