fix: allow clearing all period discounts from promo groups

Empty period_discounts dict was normalized to None by the schema,
making it indistinguishable from "field absent" (don't update).
Now empty dict passes through to CRUD which correctly sets
period_discounts=None in DB, clearing all discounts.

app/database/crud/promo_group.py

@@ -166,7 +166,7 @@ async def update_promo_group(
         group.device_discount_percent = max(0, min(100, device_discount_percent))
     if period_discounts is not None:
         normalized_period_discounts = _normalize_period_discounts(period_discounts)
-        group.period_discounts = normalized_period_discounts or None
+        group.period_discounts = normalized_period_discounts if normalized_period_discounts else None
     if auto_assign_total_spent_kopeks is not None:
         value = max(0, auto_assign_total_spent_kopeks)
         group.auto_assign_total_spent_kopeks = value if value > 0 else None

app/webapi/schemas/promo_groups.py

@@ -18,7 +18,9 @@ def _normalize_period_discounts(value: dict[object, object] | None) -> dict[int,
             except (TypeError, ValueError):
                 continue
 
-    return normalized or None
+    # Return empty dict (not None) so the backend can distinguish
+    # "clear all discounts" ({}) from "don't touch discounts" (None/absent).
+    return normalized
 
 
 class PromoGroupResponse(BaseModel):