Prevent error when using --api & comments have no example code.

Output if trying to use `--api` flag on anything that doesn't have example code in the comments:


```
/usr/local/share/npm/lib/node_modules/dox/lib/api.js:25
  code.forEach(function(block){
       ^
TypeError: Cannot call method 'forEach' of null
    at Object.module.exports [as api] (/usr/local/share/npm/lib/node_modules/dox/lib/api.js:25:8)
    at Socket.<anonymous> (/usr/local/share/npm/lib/node_modules/dox/bin/dox:46:30)
    at Socket.EventEmitter.emit (events.js:93:17)
    at Pipe.onread (net.js:418:51)
```

Simply needs to ensure `code` is an array before doing the `forEach`. Patch includes previously failing test.