Make MapToolServer's ID explicitly nullable

ServiceAnnouncer requires a non-null String, but an empty server ID is
not well-formed so a ServiceAnnouncer shouldn't be created.

This was previously handled by MapToolServer being passed null config
and the absence of a port to give was sufficient to prevent creating a
ServiceAnnouncer with an empty ID,
but we can be more defensive about this and check for a null pointer.

src/main/java/net/rptools/maptool/client/MapTool.java

@@ -185,7 +185,7 @@ public class MapTool {
       var campaign = CampaignFactory.createBasicCampaign();
       var policy = new ServerPolicy();
 
-      server = new MapToolServer("", new Campaign(campaign), null, false, policy, playerDB);
+      server = new MapToolServer(null, new Campaign(campaign), null, false, policy, playerDB);
       client = new MapToolClient(server, campaign, playerDB.getPlayer(), connections.clientSide());
     } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
       throw new RuntimeException("Unable to create default personal server", e);

src/main/java/net/rptools/maptool/server/MapToolServer.java

@@ -91,7 +91,7 @@ public enum State {
   private State currentState;
 
   public MapToolServer(
-      String id,
+      @Nullable String id,
       Campaign campaign,
       @Nullable ServerConfig config,
       boolean useUPnP,
@@ -103,7 +103,7 @@ public MapToolServer(
     this.playerDatabase = playerDb;
 
     this.announcer =
-        config == null
+        config == null || id == null
             ? null
             : new ServiceAnnouncer(id, config.getPort(), AppConstants.SERVICE_GROUP);