Pass a default value to the first param of bp_get_the_body_class()

When bp_get_the_body_class() is run as a filter on 'body_class', the first
param is never empty. However, if you call the function directly, or in
particular if you call the echo wrapper bp_the_body_class(), you will get a
PHP error due to the fact that no default value for the function has been
provided. This changeset prevents any such errors.

Fixes #5025

Props tivnet

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7113 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
When leaving the "What's New" field, shrink if it's empty

Fixes #4364

Props SGr33n

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7114 cdf35c40-ae34-48e0-9cc9-0c9da1808c22