vue-server-renderer 2.3.1 on Node.js NPM

#5536 fix regression when render function returns null (@gebilaoxiong via #5539)
#5540 vue-server-renderer/server-plugin: ensure assets are unique (@pi0 via #5540)
#5553 vue-server-renderer/server-plugin: avoid swallowing webpack error when entry is not found
#5559 vue-server-renderer: with runInNewContext: false, the bundle is now indeed executed in the same context. The 2.3.0 behavior is still available behind a new option value runInNewContext: 'once'. See docs for more details.