empty opts if it doesn't exist to prevent crashing

If there isn't a `package.json`, jadeify crashes because `opts` is null:

```
$ browserify -t jadeify rawr.js

/tmp/node_modules/jadeify/lib/jadeify.js:27
                opts.filename = fileName;
                              ^
TypeError: Cannot set property 'filename' of null
    at /tmp/node_modules/jadeify/lib/jadeify.js:27:31
    at /tmp/node_modules/jadeify/lib/jadeify.js:62:20
    at testDir (/tmp/node_modules/jadeify/node_modules/find-parent-dir/index.js:20:36)
    at /tmp/node_modules/jadeify/node_modules/find-parent-dir/index.js:26:7
    at Object.cb [as oncomplete] (fs.js:168:19)
```

This patch keeps jadeify from crashing.