Use strict type checking when setting default 'include' value in BP_User_Query

Using the empty() check resulted in problems when empty arrays were passed into
BP_User_Query as the 'include' parameter, such as when querying for a user's
friend requests when the user did not have any - in these cases, all users
ended up being returned.

Fixes #5071

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7239 cdf35c40-ae34-48e0-9cc9-0c9da1808c22