Avoid crash when back_leveling_delay is None.

issue3195
review785002
(transplanted from fcd869cb3d0c35b2aa5907ef5ad4fe0202b8a8d8)

work.py

@@ -484,7 +484,7 @@ class Work:
 
             if constraint_finish:
                 late_finish = self.add_hours(work.company, constraint_finish,
-                        -work.back_leveling_delay)
+                        -(work.back_leveling_delay or 0.0))
             else:
                 late_finish = None