Fix issue where setting StaggeredLayout.ColumnWidth to NaN would caus…

…e an exception (#4064)

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## Fixes #4063 
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The exception was caused by having ``NaN`` the value of width in ``item.Element.Measure``. This PR fixes this by adding a check if ``DesiredColumnWidth`` is ``NaN``, and if so setting it instead to the available width.

## PR Type

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Bugfix
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## What is the current behavior?
When ``DesiredColumnWidth`` is ``NaN``,  an exception will be thrown when measuring item sizes, which causes a crash or will freeze the app.

## What is the new behavior?
If ``DesiredColumnWidth`` is ``NaN``, items are stretched to fit the available width.
Example:
![image](https://user-images.githubusercontent.com/59544401/120867269-6eaac500-c546-11eb-91e1-a2caad418212.png)


## PR Checklist

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- [x] Contains **NO** breaking changes

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## Other information

Microsoft.Toolkit.Uwp.UI.Controls.Primitives/StaggeredLayout/StaggeredLayout.cs

@@ -147,14 +147,16 @@ protected override Size MeasureOverride(VirtualizingLayoutContext context, Size
             double availableWidth = availableSize.Width;
             double availableHeight = availableSize.Height;
 
-            double columnWidth = Math.Min(DesiredColumnWidth, availableWidth);
+            // This ternary prevents the column width from being NaN, which would otherwise cause an exception when measuring item sizes
+            double columnWidth = double.IsNaN(DesiredColumnWidth) ? availableWidth : Math.Min(DesiredColumnWidth, availableWidth);
             if (columnWidth != state.ColumnWidth)
             {
                 // The items will need to be remeasured
                 state.Clear();
             }
 
-            state.ColumnWidth = Math.Min(DesiredColumnWidth, availableWidth);
+            // This ternary prevents the column width from being NaN, which would otherwise cause an exception when measuring item sizes
+            state.ColumnWidth = double.IsNaN(DesiredColumnWidth) ? availableWidth : Math.Min(DesiredColumnWidth, availableWidth);
             int numColumns = Math.Max(1, (int)Math.Floor(availableWidth / state.ColumnWidth));
 
             // adjust for column spacing on all columns expect the first