Sending undef string with {add} adds whole library

MPD protocol specifies that an empty string adds the whole library (for historical reasons "/" also works).
Net::MPD on the other hand also reacts this way if a value is not initialized at all.

Code example:

```
#usr/bin/perl

use Net::MPD; Net::MPD->connect($ENV{MPD_HOST})->add(undef);          
```