_me cannot be used to ask "is this user a member of this Space?"

Summary: It seems intuitively obvious that I should be able to look at _me and see if the current user is local or not. But that doesn't work, because _me throws a Warning, and there is no way to recover from it.

I'm adding a warnIfNotLocal parameter to _me(). This defaults to true (to preserve current behavior, and because it's the right answer unless you know what you are doing), but you can set it to false to get Empty instead.