Fix LayerVisitor stopping early instead of skipping covered columns.

worldedit-core/src/main/java/com/sk89q/worldedit/function/visitor/LayerVisitor.java

@@ -99,7 +99,7 @@ public class LayerVisitor implements Operation {
 
             // Abort if we are underground
             if (function.isGround(column.toBlockVector3(maxY + 1))) {
-                return null;
+                continue;
             }
 
             boolean found = false;