fix: check for nil body rather than falsey body when determining how …

…to render mocked response

Fixes: #99

lib/pact/mock_service/request_handlers/interaction_replay.rb

@@ -182,7 +182,7 @@ def self.response_from response
         end
 
         def self.render_body body
-          return '' unless body
+          return '' if body.nil?
           body.kind_of?(String) ? body.force_encoding('utf-8') : body.to_json
         end
       end