Update CalendarViewContent closures to accept nil return values

[calda]

This PR updates the closures on CalendarViewContent to accept nil return values. This makes it easier for consumers to handle unexpected cases.

For example, today using [weak self] with CalendarViewContent is tricky:

CalendarViewContent(
  visibleDateRange: Date.distantPast...Date.distantFuture, 
  monthsLayout: .vertical)
  .dayItemProvider { [weak self] day in
    guard let self else { 
      // We have to return a non-optional value, but can't.
      // The only option is to crash.
      preconditionFailure("self unexpectedly deallocated already") 
    }
        
    return self.dayItemProvider(day)
  }

after these changes its much easier:

CalendarViewContent(
  visibleDateRange: Date.distantPast...Date.distantFuture, 
  monthsLayout: .vertical)
  .dayItemProvider { [weak self] day in
    guard let self else { return nil }
    return self.dayItemProvider(day)
  }

[calda]

To avoid having to change any of the existing call sites, we can have the instance properties still always return a non-nil value, even if the user-provided closure returns nil. An easy way to do this is to just fall back to the existing default item provider, which always returns a valid value.

[bryankeller]

Awesome, thanks @calda !