Taglib segfaults - Null String

Recently I faced up to some crashes due to metadata extraction... mainly when I'm trying to read a lot of embedded_covers at the same time. I realized that Mixxx is getting the info directly from the tag and trying to convert to QString and it was also the cause of the crashes.

Like this

```C++
if (tstring == TagLib::String::null) {
    return QString();
}
return TStringToQString(tstring);
```

I don't know if this is in the coding guidelines but usually in mixxx we exit function early only when an error occurs. The normal *everything went ok* exist is always at the end. In this case `tstring` being `null` is the error