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 commit 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.
Merge pull request #36 from Minecrell/patch-1

Correctly handle wrapped server pings with no favicon.