In update_meta() functions, return integer value if a new metadata is created

Previously, BP update_meta() functions returned true on a successful update
*or* on the successful creation of new metadata. This is not consistent with
WP's update_metadata(), which on the creation of new metadata acts as a wrapper
for add_metadata(), which in turn returns an integer (the ID of the newly
created database row).

This changeset aligns BP's behavior with WP's.

We don't have any tests that directly address the data type returned by these
functions, but we do have a number of assertTrue assertions to verify the
setup of various tests. Where appropriate, these have been changed to the more
generous assertNotEmpty.

See #5399

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8132 cdf35c40-ae34-48e0-9cc9-0c9da1808c22