undefined key throws

From the readme:

```
//undefined obj or key returns undefined, unless a default is supplied
delve(undefined, 'a.b.c') === undefined;
delve(undefined, 'a.b.c', 'foo') === 'foo';
delve(obj, undefined, 'foo') === 'foo';
```

Start with the README examples and the dlv function entry point to reproduce the undefined-key case. Make the behavior match the documented default-value examples, then run the existing test suite; the payload does not name a specific test file.

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