zPermissions: Return empty string instead of null when prefix/suffix is unset

I've been doing my testing of the zPerms/Vault chat interface with Herochat, but apparently there are chat plugins that choke when getPlayerPrefix() etc. return null.

I wasn't originally sure whether null was a valid return value for Vault Chat API methods. But I think I'll play it safe and never return null for prefix/suffix. (I can't make that change on my side since null means the metadata/info node is unset, vs. set as empty string.)

Anyway, thanks again!