Fails confusingly if a plugin's load() method returns a buffer

[callumlocke]

I made an ad hoc plugin with a load() method. From this load() method, I returned a buffer in some cases, forgetting to call .toString() before returning (common mistake).

Rollup should probably throw a TypeError if a plugin returns anything except a string/null/undefined.

But strangely, when I return a buffer, rollup behaves as if I had returned a string, i.e. it continues to the transforms... except it then passes undefined into the first transform. In my case, this led to the following error:

Cannot read property 'search' of undefined
  at transform in node_modules/rollup-plugin-inject/dist/rollup-plugin-inject.cjs.js:111:12

which comes from this line in rollup-plugin-inject:

            if ( code.search( firstpass ) == -1 ) return null;