Fix a NPE with brush using a global mask and no brush mask

src/main/java/com/sk89q/worldedit/tools/BrushTool.java

@@ -177,6 +177,7 @@ public class BrushTool implements TraceTool {
         BlockBag bag = session.getBlockBag(player);
 
         EditSession editSession = session.createEditSession(player);
+        if (mask != null) {
             Mask existingMask = editSession.getMask();
             if (existingMask == null) {
                 editSession.setMask(mask);
@@ -187,6 +188,7 @@ public class BrushTool implements TraceTool {
                 newMask.add(mask);
                 editSession.setMask(newMask);
             }
+        }
 
         try {
             brush.build(editSession, target, material, size);