Update test/mcp/server/transports/streamable_http_transport_test.rb

Co-authored-by: Koichi ITO <[email protected]>

Author: bigH

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -694,7 +694,7 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           assert_equal({ "Content-Type" => "application/json" }, response[1])
 
           body = response[2][0]
-          assert body.blank?
+          assert_nil(body)
         end
 
         test "handle post request with a standard error" do