$regex throws incorrect error with an empty string argument

In 0.7.1.2, a query such as `Foo.find({field: {$regex: "", $options: "i"}})` leads to the following error:

```
Exception from Deps recompute: Error: $options needs a $regex
```

This is because the argument checking logic doesn't account for `$regex` possibly being an empty string. In Mongo, the same query would return all documents for which the field exists.

Discovered by @geekyme in mizzao/meteor-autocomplete#15