model: geometry: Fix 'sl' setter.

src/Model/Geometry/Point.py

@@ -40,8 +40,8 @@ class Point(object):
     def sl(self, sl):
         if sl == self._profile.sl:
             self._sl = None
-
-        self._sl = sl
+        else:
+            self._sl = sl
 
     @property
     def name(self):