next.js

8e59595a - [metadata] remove dead code of metadata routes handling (#77336)

[metadata] remove dead code of metadata routes handling (#77336) ### What - `[].[ext]` was temporary convention invented in a PR but we never decided to go that way. Remove the tests for it - `dynamicExtensions` can be either empty array or null, for both case it can skip adding the regex for it. This just saves some bytes for generated regex. Also added some test