fix empty string bug

My previous PR introduced an empty string bug.

It would `npm run test ''` which makes npm try to run empty string which fails terribly. It tries to run the command `''` for the local dependencies named `test`

For example `npm run tape test` would run tape tests.

This fixes the bug by never passing in `''`.

Please update & republish.

Either deprecate or unpublish 0.0.6

Apologies for introducing this bug.