Use spaces for "empty" titles.

Due to a breaking change in Spigot 1.17, sending a title with an empty string no longer works, even if a non-empty subtitle is provided. The TitleAnnouncer and TitleHealthBar components only use the subtitle portion of the title API to avoid obtrusive behavior.

With this commit, the two components send a single space character as the title. Effectively, there is no difference between showing the empty string or a single space on the client-side. It's a dirty hack to fix something that should be fixed in Spigot instead, but odds are that it won't be.

Fixes #683

src/main/java/com/garbagemule/MobArena/announce/TitleAnnouncer.java

@@ -16,7 +16,7 @@ public class TitleAnnouncer implements Announcer {
 
     @Override
     public void announce(Player player, String message) {
-        player.sendTitle("", message, fadeIn, stay, fadeOut);
+        player.sendTitle(" ", message, fadeIn, stay, fadeOut);
     }
 
 }

src/main/java/com/garbagemule/MobArena/healthbar/TitleHealthBar.java

@@ -27,7 +27,7 @@ class TitleHealthBar implements HealthBar {
         String message = title.isEmpty() ? health : title + " " + health;
 
         players.forEach(player -> player.sendTitle(
-            "",
+            " ",
             message,
             FADE_IN_TICKS,
             STAY_TICKS,