Add Nullable private dependency.

Author: sharpjs

Directory.Packages.props

@@ -15,7 +15,8 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageVersion Include="Subatomix.Testing"                           Version="3.2.0" />
+    <PackageVersion Include="Nullable"          Version="1.3.1" />
+    <PackageVersion Include="Subatomix.Testing" Version="3.2.0" />
   </ItemGroup>
 
   <ItemGroup>
@@ -24,7 +25,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(IsTestProject)' != 'true'">
-    <GlobalPackageReference Include="Microsoft.SourceLink.GitHub"         Version="8.0.0" />
+    <GlobalPackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
   </ItemGroup>
 
 </Project>

Prequel/Prequel.csproj

@@ -11,4 +11,8 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Nullable" PrivateAssets="all" />
+  </ItemGroup>
+
 </Project>