db.get returns the entire store if key is 0

Similar to issue #1. To reproduce:

```
const JSONStore = require('json-store')
const db = JSONStore('./index.json')

db.set(0, 'zero')
db.get('zero')
// => returns { '0': 'zero' }, but should return 'zero'
```

The cause is [this line](https://github.com/juliangruber/json-store/blob/6e53a83af1e61fec9fa5dfcf19f161df1cf0d61b/index.js#L10), which checks if `key` is truthy. Instead, it should explicitly check whether `key` is defined.