More precise error checking in `bp_sort_by_key()`.

In order to support sorting with a value of 0, we must use `isset()` to verify
the existence of a value.

See #6047.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9192 cdf35c40-ae34-48e0-9cc9-0c9da1808c22