Correctly handle wrapped server pings with no favicon.

In the current ProtocolLib release (3.1.0) the `getFavicon()` method will throw a NullPointerException if the server is not sending a favicon. This was partly fixed in 3c5482f but there's still no way of checking if the server is sending a favicon, without checking if the encoded output of the `CompressedImage.toEncodedText()` will return a valid result.

This pull request will make the favicon getter and setter in the server ping correctly handle pings with no favicon, by returning null for `getFavicon()` (if no favicon will be displayed) and allowing to hide the favicon by setting the favicon to null.