check if dir and file exists before generate bundle (#236)

Solves for my own case darionco/rollup-plugin-web-worker-loader#4

Which generates a new configuration for the worker which, when inlined, has not file or directory and rollup-plugin-postcss tries to compile it and crashes

src/index.js

@@ -102,7 +102,10 @@ export default (options = {}) => {
     },
 
     async generateBundle(opts, bundle) {
-      if (extracted.size === 0) return
+      if (
+        extracted.size === 0 ||
+        !(opts.dir || opts.file)
+      ) return
 
       // TODO: support `[hash]`
       const dir = opts.dir || path.dirname(opts.file)