Fix HelpCommand searching

Prevent index out of bounds exception when finding possible matches in
the help topics.
A general empty string is caught and directed to the defaultTopic by
getHelpTopic(). #findPossibleMatches hence should not concern itself
with possible empty strings anymore.

paper-api/src/main/java/org/bukkit/command/defaults/HelpCommand.java

@@ -143,6 +143,7 @@ protected HelpTopic findPossibleMatches(@NotNull String searchString) {
             searchString = searchString.substring(1);
         }
 
+        if (searchString.isEmpty()) return null; // Paper - prevent index out of bounds - nothing matches an empty search string, should have been special cased to defaultTopic earlier, just return null.
         for (HelpTopic topic : Bukkit.getServer().getHelpMap().getHelpTopics()) {
             String trimmedTopic = topic.getName().startsWith("/") ? topic.getName().substring(1) : topic.getName();
 

paper-api/src/main/java/org/bukkit/help/IndexHelpTopic.java

@@ -49,10 +49,15 @@ public boolean canSee(@NotNull CommandSender sender) {
         if (sender instanceof ConsoleCommandSender) {
             return true;
         }
-        if (permission == null) {
-            return true;
+        // Paper start - Fix HelpCommand searching - do not show index if no topic is visible to the sender
+        if (permission != null && !sender.hasPermission(permission)) return false; // old spigot permission check
+
+        for (HelpTopic topic : allTopics) {
+            if (topic.canSee(sender)) return true;
         }
-        return sender.hasPermission(permission);
+
+        return false;
+        // Paper end - Fix HelpCommand searching - do not show index if no topic is visible to the sender
     }
 
     @Override