Add guard against division by zero in target_shifts calculation with warning logging

rota_generator/shifts.py

@@ -2007,11 +2007,24 @@ class RotaBuilder(object):
                         for i in shift.sites
                     )
 
-                    target_shifts = (
-                        total_shifts
-                        / full_time_equivalent_joined
-                        * worker.get_fte(shift=shift.name)
-                    )
+                    # Guard against division by zero: if no full-time
+                    # equivalent is available for the shift's sites the
+                    # denominator may be zero (for example when all site
+                    # FTEs are 0). In that case emit a warning and set the
+                    # target to 0 rather than raising an exception.
+                    if not full_time_equivalent_joined:
+                        # avoid ZeroDivisionError
+                        self.add_warning(
+                            "Zero FTE for sites",
+                            f"full_time_equivalent sum is zero for shift {shift.name}; setting target_shifts=0 for worker {worker.name}",
+                        )
+                        target_shifts = 0
+                    else:
+                        target_shifts = (
+                            total_shifts
+                            / full_time_equivalent_joined
+                            * worker.get_fte(shift=shift.name)
+                        )
 
                     if self.use_previous_shifts:
                         if shift.name in worker.previous_shifts: