Restore nullability for displayName in UpsertPlayerInfo (#1172)

This makes the UpsertPlayerInfo's displayName truly nullable as before the ComponentHolder was introduced.

proxy/src/main/java/com/velocitypowered/proxy/tablist/VelocityTabList.java

@@ -110,8 +110,12 @@ public class VelocityTabList implements InternalTabList {
       if (!Objects.equals(previousEntry.getDisplayNameComponent().orElse(null),
           entry.getDisplayNameComponent().orElse(null))) {
         actions.add(UpsertPlayerInfo.Action.UPDATE_DISPLAY_NAME);
-        playerInfoEntry.setDisplayName(new ComponentHolder(player.getProtocolVersion(),
+        playerInfoEntry.setDisplayName(entry.getDisplayNameComponent().isEmpty()
-              entry.getDisplayNameComponent().get()));
+                ?
+                null :
+                new ComponentHolder(player.getProtocolVersion(),
+                        entry.getDisplayNameComponent().get())
+        );
       }
       if (!Objects.equals(previousEntry.getLatency(), entry.getLatency())) {
         actions.add(UpsertPlayerInfo.Action.UPDATE_LATENCY);
@@ -140,8 +144,12 @@ public class VelocityTabList implements InternalTabList {
       playerInfoEntry.setProfile(entry.getProfile());
       if (entry.getDisplayNameComponent().isPresent()) {
         actions.add(UpsertPlayerInfo.Action.UPDATE_DISPLAY_NAME);
-        playerInfoEntry.setDisplayName(new ComponentHolder(player.getProtocolVersion(),
+        playerInfoEntry.setDisplayName(entry.getDisplayNameComponent().isEmpty()
-              entry.getDisplayNameComponent().get()));
+                ?
+                null :
+                new ComponentHolder(player.getProtocolVersion(),
+                        entry.getDisplayNameComponent().get())
+        );
       }
       if (entry.getChatSession() != null) {
         actions.add(UpsertPlayerInfo.Action.INITIALIZE_CHAT);

proxy/src/main/java/com/velocitypowered/proxy/tablist/VelocityTabListEntry.java

@@ -80,7 +80,11 @@ public class VelocityTabListEntry implements TabListEntry {
     this.displayName = displayName;
     UpsertPlayerInfo.Entry upsertEntry = this.tabList.createRawEntry(this);
     upsertEntry.setDisplayName(
-        new ComponentHolder(this.tabList.getPlayer().getProtocolVersion(), displayName));
+            displayName == null
+                    ?
+                    null :
+                    new ComponentHolder(this.tabList.getPlayer().getProtocolVersion(), displayName)
+    );
     this.tabList.emitActionRaw(UpsertPlayerInfo.Action.UPDATE_DISPLAY_NAME, upsertEntry);
     return this;
   }