Handle missing body variable

As of Rack `3.1.0`, `rack.input` is now optional, so cannot always be
guaranteed to be present (https://github.com/rack/rack/blob/b4c92944e0dd04874bed36281fc8e1a44023677f/CHANGELOG.md?plain=1#L40)

Author: pezholio

lib/pact/consumer/mock_service/rack_request_helper.rb

@@ -24,7 +24,7 @@ def request_as_hash_from env
         end
 
         request[:headers] = headers_from env
-        body_string = request[:body].read
+        body_string = request[:body]&.read || ""
 
         if body_string.empty?
           request.delete :body