Using bbp_get_displayed_user_field() when there is no displayed user causes debug notices

[johnjamesjacoby]

If you call bbp_get_displayed_user_field() when not viewing a user, or if you attempt to get a field that does not exist, you'll be greeted with debug notices because the object being referenced is a stdObject, not a WP_User object.

If we use WP_User objects, we'll inherit the magic methods for free, and avoid these problems completely.

Worth noting here that we can do the same for the 4 main query loops.

[johnjamesjacoby]

In 4996:

Replace generic stdClass usages with correct object types.

Fixes debug notices with bbp_get_displayed_user_field() when requesting non-existing fields or when there is no displayed user, by allowing the magic get() methods of the WP_User class to bail if the field is not set.

Also prevents similar issues with main query loops. Fixes #2359.