Show the example a warning occurred in if nil call site is given.

Sometimes we issue warnings outside of a user's spec, but the warning was caused by something the user did.. An example of this is in: https://github.com/rspec/rspec-mocks/pull/496 where we want to tell the user where the error came from, but we can't really detect where the object was frozen.

This prints that if a nil call site is given.