Corrections to RectHelper and SizeHelper's definition of "empty"

[mcooley]

RectHelper and SizeHelper use a definition of "Empty" which matches .NET's System.Windows.Rect.Empty. A Rect or Size with Width == 0 or Height == 0 is not "empty" because it is not the Empty value.

• RectHelper.Empty and SizeHelper.Empty actually return structs where the Width and Height are initialized to negative infinity, not 0.
• For GetLeft/GetTop/GetRight/GetBottom, the implementation actually calls GetIsEmpty rather than checking for positive Width or Height.
• In the Remarks section for RectHelper.GetIsEmpty and SizeHelper.GetIsEmpty, add the same caveat that is in the .NET docs: "this method should not be used to check for zero area".

Note that this information also applies to the Microsoft.UI.Xaml version of these methods. I assume I'll need to open a separate PR against winapps-winrt-api for that; please let me know if there's a different process which is preferred.

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[ShannonLeavitt]

@jwmsft

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[jwmsft]

@mcooley, thanks for these fixes. I'll take care of copying them over to Microsoft.UI.Xaml so you won't need to worry about it.

sign-off

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[jwmsft]

sign-off

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