Fixed unnecessary zero-length array allocations

Changed all currently present zero-length array allocations in the codebase to use `Array.Empty` instead.

OpenRA.Mods.Common/Activities/Air/Land.cs

@@ -54,7 +54,7 @@ namespace OpenRA.Mods.Common.Activities
 			aircraft = self.Trait<Aircraft>();
 			this.target = target;
 			this.offset = offset;
-			this.clearCells = clearCells ?? new CPos[0];
+			this.clearCells = clearCells ?? Array.Empty<CPos>();
 			this.landRange = landRange.Length >= 0 ? landRange : aircraft.Info.LandRange;
 			this.targetLineColor = targetLineColor;