Fix versioning when `runtime` is `'node-webkit'` but `target` is empty

In my commit 53f3e38e1619eb58e55037d335d91fb7bcd62fa3 I accidentally neglected the case when `target` is empty.

This pull request is expected to fix mapbox/node-sqlite3#339 and a bunch of other similar node-webkit-related issues occuring in addon-containing modules that use node-pre-gyp.