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/UpdatePath.cs

@@ -81,6 +81,7 @@ namespace OpenRA.Mods.Common.UpdateRules
 				new RenameEmitInfantryOnSell(),
 				new SplitRepairDecoration(),
 				new MoveHackyAISupportPowerDecisions(),
+				new DefineGroundCorpseDefault(),
 			})
 		};