Add undefined fallback for valueMonth state

Added else clauses to set valueMonth to undefined when month values are not provided:
- Prevents stale date values in calendar
- Ensures calendar displays correctly when switching between date ranges

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/shared/ui/calendar/nice-calendar-month.tsx

@@ -55,6 +55,8 @@ const NiceCalendarMonth = ({
         if (startDate.isValid()) {
           setValueMonth(startDate.toDate());
         }
+      } else {
+        setValueMonth(undefined);
       }
     }
     else if(calendarType === CalendarType.End){
@@ -71,6 +73,8 @@ const NiceCalendarMonth = ({
         if (endDate.isValid()) {
           setValueMonth(endDate.toDate());
         }
+      } else {
+        setValueMonth(undefined);
       }
     }
     else if(calendarType === CalendarType.Single){
@@ -79,6 +83,8 @@ const NiceCalendarMonth = ({
         if (singleDate.isValid()) {
           setValueMonth(singleDate.toDate());
         }
+      } else {
+        setValueMonth(undefined);
       }
     }
   };