Empty password support

Hello again! 

As per the title.

It should be noted however that if support for rekeying the DB is ever added the [following notes](https://www.zetetic.net/sqlcipher/sqlcipher-api/#sqlcipher_export) should be accounted for. You cannot rekey a plain text DB (ie, that originally had an empty password set).
I only mention the above as I might look at adding rekeying to the DB soon...!

This aids in debugging allowing the SQLite DB file to be check if needed:

``` objectivec
#ifdef DEBUG
   NSDictionary *opts = nil;
#else
   NSDictionary *opts = @{EncryptedStorePassphraseKey : password};
#endif
// DB will now only be encrypted for the Release configuration
```

Cheers,

Rich