Actually, this conditional seems entirely wrong to me.  Isn't the goal here to allow a warning that explicitly passes `:call_site => nil` in order to prevent a call site from showing up at all?  For example, when issuing a deprecation about a command line option, there is no user call site and we use `:call_site => nil` to prevent a call site from showing up in the message since there is no useful call site.  Maybe I'm reading this wrong, but it looks like this will cause a call site to be included when an explicit `nil` has been passed...is that what we want?