[Bleeding] Don't allow null display names. Fixes BUKKIT-5254

Currently, plugins can set a player's display name to null, which could
cause issues for other calls to getDisplayName that aren't expecting a null
value. This changes setDisplayName to follow the same logic as
setPlayerListName, which sets the player's name back to their unmodified
"vanilla" name if it receives a null value as a parameter.

(cherry picked from commit 04c2dcf97e29821eeec3537b470058238a5fa8d1)