Fixes an exception when trying to load an empty book list

src/main/java/net/knarcraft/bookswithoutborders/BooksWithoutBorders.java

@@ -139,7 +139,12 @@ public class BooksWithoutBorders extends JavaPlugin {
                     getInstance().playerLetterIndex.put(playerUUID, BookFileHelper.populateLetterIndices(newFiles));
                 }
             }
-            return new ArrayList<>(getInstance().playerBooksList.get(playerUUID));
+            List<String> books = getInstance().playerBooksList.get(playerUUID);
+            if (books != null) {
+                return new ArrayList<>(books);
+            } else {
+                return new ArrayList<>();
+            }
         } else {
             return new ArrayList<>();
         }