Return an empty string from get_meta() functions when no value is found

BP get_meta() functions have been inconsistent about the type of value returned
when no match is found for the object_id+meta_key combination. In some cases,
we followed WP's get_metadata() and returned an empty string. In others, we
returned false.

This changeset aligns all of our get_meta() functions with WP in this regard,
returning an empty string when no matching value is found. A boolean false is
still returned when invalid arguments are passed to the functions.

Relevant unit tests have been updated, including strict assertSame() checking
where appropriate.

See #5399

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8131 cdf35c40-ae34-48e0-9cc9-0c9da1808c22