TypeError in getDeepBounds

`Generator.prototype.getDeepBounds` can reasonably return `undefined` in the case of an empty layer group. In certain situations (as demonstrated, e.g., in adobe-photoshop/generator-assets-automation#8), a recursive call that results in `undefined` can cause a `TypeError`. This fixes that error, but continues to allow `getDeepBounds` to return `undefined`.

Addresses adobe-photoshop/generator-assets#244.