Fix to NPE in the Party API getPartyName method when the target isn't in a party

When using the Party API, if you request the party name of a player that isn't in a party using getPartyName, a NPE is thrown, as the getParty() method on the mcMMO player returns a null, meaning that the getName() method cannot be called.

This patch causes the method to return null instead if the player isn't in a party, rather than throw an NPE.