@neodx/svg Alert on empty input

[secundant]

Users may inadvertently select the wrong source folder and become confused after a build that completes without any notification and visible changes.
For example, this config

// vite.config.ts
export default defineConfig({
  svg({
    root: 'broken-path'
  })
});

will yield to no result.

The solution is simple: we should display an alert with information about a possible mistake in the root path.