Check that the response is not empty before calling html_request? in should_inject_xray

`html_request?(headers, response)` will fail to run (crash on `response.body.include?("<html")`) when called on a response that is an array or doesn't have a body for any reason.

I moved the check for response emptiness to run before html_request?

Maybe `empty?` should be called in the `html_request?` method itself?