ResolveNuGetPackageAssets will throw NullReferenceException when PackageVersion is not set

[donnie-msft]

Moved from https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1896623

I have a legacy .csproj solution that uses Central Package Management. If I add a to a project and exclude the Version property (as expected for CPM) but forget to define the Version in Directory.Packages.props for that package ID, I get a null reference exception:

20>C:\Program Files\Microsoft Visual Studio\2022\IntPreview\MSBuild\Microsoft\NuGet\17.0\Microsoft.NuGet.targets(198,5): error : Object reference not set to an instance of an object.

NU1010: The PackageReference items RuntimeContracts do not have corresponding PackageVersion.

Related: #77