Default the optional effect sequence to null

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

@@ -0,0 +1,39 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2020 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.Collections.Generic;
+
+namespace OpenRA.Mods.Common.UpdateRules.Rules
+{
+	class SpawnActorPowerDefaultEffect : UpdateRule
+	{
+		public override string Name { get { return "Set SpawnActorPower EffectSequence to it's previous default value."; } }
+		public override string Description
+		{
+			get
+			{
+				return "The 'EffectSequence' of 'SpawnActorPower' is unset by default.";
+			}
+		}
+
+		public override IEnumerable<string> UpdateActorNode(ModData modData, MiniYamlNode actorNode)
+		{
+			foreach (var spawnActorPower in actorNode.ChildrenMatching("SpawnActorPower"))
+			{
+				var effectNode = spawnActorPower.LastChildMatching("EffectSequence");
+				if (effectNode == null)
+					spawnActorPower.AddNode("EffectSequence", "idle");
+			}
+
+			yield break;
+		}
+	}
+}