Do not break lines on empty callbacks

Boxes.cc() did move the coordinate system arround even if the callback is
None. This breaks the continuity of the border. If there is callback the
line is still broken.

boxes/__init__.py

@@ -430,26 +430,23 @@ class Boxes:
         """
         if y is None:
             y = self.burn
-        self.ctx.save()
-        self.moveTo(x, y)
-        if callable(callback):
+
+        if hasattr(callback, '__getitem__'):
+            try:
+                callback = callback[number]
+                number = None
+            except (KeyError, IndexError):
+                pass
+
+        if callback and callable(callback):
+            self.ctx.save()
+            self.moveTo(x, y)
             if number is None:
                 callback()
             else:
                 callback(number)
-
-        elif hasattr(callback, '__getitem__'):
-            try:
-                callback = callback[number]
-                if callable(callback):
-                    callback()
-            except (KeyError, IndexError):
-                pass
-            except:
-                self.ctx.restore()
-                raise
-            
-        self.ctx.restore()
+            self.ctx.restore()
+            self.ctx.move_to(0, 0)
 
     def getEntry(self, param, idx):
         """