Perf issue fix: avoid creating an inject plugin when there are no shims to inject

[jlas]

I started using vite-plugin-node-polyfills on a fairly large vite project and it made vite build slower by 50-100%. It also caused increased memory usage (causing OOM errors).

I tracked down the source of the slowdown to the inject plugin. My config had globals.{Buffer, global, process} all set to false so the plugin was not necessary.

I reworked the logic to avoid passing in the inject plugin when there are no shims to inject.