Enhancement: Validate parameters for nil on noir.util.crypt's compare

Calling:

```
(compare nil (encrypt "hello"))
```

returns false as expected. However, calling

```
(compare "hello" nil)
```

raises a NullPointerException from org.mindrot.jbcrypt.BCrypt.hashpw.   It would be better to validate both parameters before passing them down to BCrypt/checkpw, and return false if one of them is nil.
