Change LinkedIn::Mash#all to return [] instead of nil when there are no results

If I call a query method that usually returns an array of results, but no results are found, LinkedIn::Mash.all currently returns nil. It makes more sense logically for it to return an empty array. With this patch, you no longer have to add handle the possiblility of `nil`, such as in this example:
```
client = LinkedIn::Client.new(...)
client.network_updates.all || []
```