Add null check before iterating over iterator.

By: Xor Boole <mcyoung@mit.edu>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaBook.java

@@ -99,11 +99,13 @@ class CraftMetaBook extends CraftMetaItem implements BookMeta {
         setTitle(SerializableMeta.getString(map, BOOK_TITLE.BUKKIT, true));
 
         Iterable<?> pages = SerializableMeta.getObject(Iterable.class, map, BOOK_PAGES.BUKKIT, true);
+        if(pages != null) {
             for (Object page : pages) {
                 if (page instanceof String) {
                     addPage((String) page);
                 }
             }
+        }
         
         generation = SerializableMeta.getObject(Integer.class, map, GENERATION.BUKKIT, true);
     }