Fix null error when world name for non-player placeholders not specified

Author: benwoo1110

src/main/java/org/mvplugins/multiverse/core/PlaceholderExpansionHook.java

@@ -113,7 +113,7 @@ public boolean persist() {
                 return Option.of(player.getWorld().getName());
             } else {
                 warning("You must specify a world name for non-player placeholders");
-                return null;
+                return Option.none();
             }
         }