Add ArgumentNullThrowHelper shared source reference

Author: captainsafia

src/Logging.AzureAppServices/src/Microsoft.Extensions.Logging.AzureAppServices.csproj

@@ -31,6 +31,7 @@
 
   <ItemGroup>
     <Compile Include="$(SharedSourceRoot)ThrowHelpers\ArgumentThrowHelper.cs" LinkBase="Shared" />
+    <Compile Include="$(SharedSourceRoot)ThrowHelpers\ArgumentNullThrowHelper.cs" LinkBase="Shared" />
     <Compile Include="$(SharedSourceRoot)CallerArgument\CallerArgumentExpressionAttribute.cs" LinkBase="Shared" />
   </ItemGroup>