fix(calendar): Fix possibly failing assert

src/Uno.UI/Microsoft/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs

@@ -530,7 +530,7 @@ private Size base_MeasureOverride(Size availableSize)
 					var itemSize = _layoutStrategy.GetElementMeasureSize(ElementType.ItemContainer, index, renderWindow); // Note: It's actually the same for all items
 					var itemBounds = _layoutStrategy.GetElementBounds(ElementType.ItemContainer, index + StartIndex, itemSize, layout, renderWindow);
 
-					if (itemSize.Width < _minCellSize.Width && itemSize.Height < _minCellSize.Height)
+					if (itemSize.Width < _minCellSize.Width && itemSize.Height < _minCellSize.Height || Cols == 0 || Rows == 0)
 					{
 						// We don't have any valid cell size yet (This measure pass has been caused by DetermineTheBiggestItemSize),
 						// so we stop right after having inserted the first child in the Children collection.