Fix an error while running HttpHelperTest

The following error is shown on running "with insecure in transport
section" test while it doesn't affect to the test result.

  error: Async::Task [oid=0x2aac31a3b4f4] [pid=93890] [2021-02-05 18:48:20 +0900]
        |   NoMethodError: undefined method `status' for nil:NilClass
        |   → /home/aho/Projects/Fluentd/fluentd/test/plugin_helper/test_http_server_helper.rb:135 in `block in start_https_request'
        |     /home/aho/Projects/Fluentd/fluentd/vendor/bundle/ruby/2.6.0/gems/async-1.28.5/lib/async/task.rb:265 in `block in make_fiber'

test/plugin_helper/test_http_server_helper.rb

@@ -132,7 +132,9 @@ def start_https_request(addr, port, verify: true, cert_path: nil, selfsigned: tr
         error = e
       end
 
-      resp = Response.new(response.status.to_s, response.body.read, response.headers)
+      if response
+        resp = Response.new(response.status.to_s, response.body.read, response.headers)
+      end
     end
 
     if error