Speed up permission recalculation.

Based on ideas in PR #102, but rewritten for correctness.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/permissions/Permission.java

@@ -52,6 +52,7 @@ public class Permission {
     }
 
     public Permission(String name, String description, PermissionDefault defaultValue, Map<String, Boolean> children) {
+        Validate.notNull(name, "Name cannot be null");
         this.name = name;
         this.description = (description == null) ? "" : description;