fix(calendar range): handle unset date and selectedValue

In the `NbCalendarRangeDayCellComponent` `date` property could be
set to `null` in case it's from bounding month and user set
`boundingMonth` to false.
In all `NbCalendarRange*CellComponent` selectedValue could be defined
but has no `start` property set. It happens because users are setting
`[range]` input on `NbCalendarRangeComponent` to `{}` so `selectedValue`
is truthy.

src/framework/theme/components/calendar/calendar-range-day-cell.component.ts

@@ -91,7 +91,7 @@ export class NbCalendarRangeDayCellComponent<D> extends NbBaseCalendarRangeCell<
     }
 
     if (this.selectedValue) {
-      return this.dateService.isSameDay(this.date, this.selectedValue.start);
+      return this.dateService.isSameDaySafe(this.date, this.selectedValue.start);
     }
   }
 

src/framework/theme/components/calendar/calendar-range-month-cell.component.ts

@@ -60,7 +60,7 @@ export class NbCalendarRangeMonthCellComponent<D> extends NbBaseCalendarRangeCel
     }
 
     if (this.selectedValue) {
-      return this.dateService.isSameMonth(this.date, this.selectedValue.start);
+      return this.dateService.isSameMonthSafe(this.date, this.selectedValue.start);
     }
   }
 

src/framework/theme/components/calendar/calendar-range-year-cell.component.ts

@@ -69,7 +69,7 @@ export class NbCalendarRangeYearCellComponent<D> extends NbBaseCalendarRangeCell
     }
 
     if (this.selectedValue) {
-      return this.dateService.isSameYear(this.date, this.selectedValue.start);
+      return this.dateService.isSameYearSafe(this.date, this.selectedValue.start);
     }
   }