Change Parachutable.GroundCorpseSequence default to null

Missions and 3rd-party mods may paradrop vehicles which normally don't need a corpse sequence (because they already have Explodes), so the old infantry-centric internal default can cause more harm than good.

OpenRA.Mods.Common/UpdateRules/Rules/DefineGroundCorpseDefault.cs

@@ -0,0 +1,68 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenRA.Mods.Common.UpdateRules.Rules
+{
+	public class DefineGroundCorpseDefault : UpdateRule
+	{
+		public override string Name { get { return "Move Parachutable GroundCorspeSequence default to yaml"; } }
+		public override string Description
+		{
+			get
+			{
+				return "Parachutable's GroundCorspeSequence 'corpse' default has been moved to yaml.";
+			}
+		}
+
+		Tuple<string, string, string, List<string>>[] fields =
+		{
+			Tuple.Create("Parachutable", "GroundCorpseSequence", "corpse", new List<string>()),
+		};
+
+		string BuildMessage(Tuple<string, string, string, List<string>> field)
+		{
+			return "The default value for {0}.{1} has been removed.\n".F(field.Item1, field.Item2) +
+				"You may wish to explicitly define `{0}: {1}` on the `{2}` trait \n".F(field.Item2, field.Item3, field.Item1) +
+				"definitions on the following actors (if they have not already been inherited from a parent).\n" +
+				UpdateUtils.FormatMessageList(field.Item4);
+		}
+
+		public override IEnumerable<string> AfterUpdate(ModData modData)
+		{
+			foreach (var field in fields)
+			{
+				if (field.Item4.Any())
+					yield return BuildMessage(field);
+
+				field.Item4.Clear();
+			}
+		}
+
+		public override IEnumerable<string> UpdateActorNode(ModData modData, MiniYamlNode actorNode)
+		{
+			foreach (var field in fields)
+			{
+				foreach (var traitNode in actorNode.ChildrenMatching(field.Item1))
+				{
+					var node = traitNode.LastChildMatching(field.Item2);
+					if (node == null)
+						field.Item4.Add("{0} ({1})".F(actorNode.Key, traitNode.Location.Filename));
+				}
+			}
+
+			yield break;
+		}
+	}
+}