Restore null page behaviour of signed books even though not strictly allowed by API

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

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

@@ -212,7 +212,7 @@ public class CraftMetaBookSigned extends CraftMetaItem implements BookMeta {
         // assert: pages != null
 
         String newText = validatePage(text);
-        pages.set(page - 1, CraftChatMessage.fromStringOrNull(newText, true));
+        pages.set(page - 1, CraftChatMessage.fromStringOrEmpty(newText, true));
     }
 
     @Override
@@ -224,7 +224,7 @@ public class CraftMetaBookSigned extends CraftMetaItem implements BookMeta {
     public void addPage(final String... pages) {
         for (String page : pages) {
             page = validatePage(page);
-            internalAddPage(CraftChatMessage.fromStringOrNull(page, true));
+            internalAddPage(CraftChatMessage.fromStringOrEmpty(page, true));
         }
     }
 

paper-server/src/main/java/org/bukkit/craftbukkit/util/CraftChatMessage.java

@@ -177,6 +177,14 @@ public final class CraftChatMessage {
         return (message == null || message.isEmpty()) ? null : fromString(message, keepNewlines)[0];
     }
 
+    public static IChatBaseComponent fromStringOrEmpty(String message) {
+        return fromStringOrEmpty(message, false);
+    }
+
+    public static IChatBaseComponent fromStringOrEmpty(String message, boolean keepNewlines) {
+        return fromString(message, keepNewlines)[0];
+    }
+
     public static IChatBaseComponent[] fromString(String message) {
         return fromString(message, false);
     }