When a render method has nothing to render, eagerly return.

By eagerly returning an empty enumerable in these cases, this avoids allocating an enumerable for the whole render method if nothing will be drawn.

OpenRA.Mods.Common/Traits/CombatDebugOverlay.cs

@@ -55,13 +55,18 @@ namespace OpenRA.Mods.Common.Traits
 		IEnumerable<IRenderable> IRenderAnnotations.RenderAnnotations(Actor self, WorldRenderer wr)
 		{
 			if (debugVis == null || !debugVis.CombatGeometry || self.World.FogObscures(self))
-				yield break;
+				return Enumerable.Empty<IRenderable>();
 
+			return RenderAnnotations(self, wr);
+		}
+
+		IEnumerable<IRenderable> RenderAnnotations(Actor self, WorldRenderer wr)
+		{
 			var blockers = allBlockers.Where(Exts.IsTraitEnabled).ToList();
 			if (blockers.Count > 0)
 			{
 				var height = new WVec(0, 0, blockers.Max(b => b.BlockingHeight.Length));
-				yield return new LineAnnotationRenderable(self.CenterPosition, self.CenterPosition + height, 1,  Color.Orange);
+				yield return new LineAnnotationRenderable(self.CenterPosition, self.CenterPosition + height, 1, Color.Orange);
 			}
 
 			var activeShapes = shapes.Where(Exts.IsTraitEnabled);