Allow empty MultiBrush with Replaceability.Any

Painting a MultiBrush previously required it to be non-empty. When
using Replaceability.Any, this is unlikely to be the desire behavior,
as some utilities (such as the upcoming LatTiler and RampTiler) may
reasonably generate an empty MultiBrush as output.

This changes painting MultiBrushes using Replaceability.Any to be
effectively no-ops. The .Tile and .Actor cases still use checks, as
these still plausibly represent mistakes.

OpenRA.Mods.Common/MapGenerator/MultiBrush.cs

@@ -633,9 +633,6 @@ namespace OpenRA.Mods.Common.MapGenerator
 				case Replaceability.None:
 					throw new ArgumentException("Cannot paint: Replaceability.None");
 				case Replaceability.Any:
-					if (this.actorPlans.Count == 0 && tiles.Count == 0)
-						throw new ArgumentException("Cannot paint: no tiles or actors");
-
 					PaintTiles(map, paintAt, finalHeightOffset, random);
 					PaintActors(map, actorPlans, paintAt);