Provide a default value for package description

SDK pack defaults the package description to the string Package Description if none is provided.

Do the same in nugetizer so that installing the package doesn't start failing for packages (maybe internal?) where a description isn't provided.

Fixes #40

src/NuGetizer.Tasks/NuGetizer.PackageMetadata.targets

@@ -15,6 +15,9 @@ Copyright (c) .NET Foundation. All rights reserved.
     <!-- Matches the SDK Pack default when set to true. -->
     <PackageId Condition="'$(IsPackable)' == 'true' and '$(PackageId)' == ''">$(AssemblyName)</PackageId>
     <PackageId Condition="'$(IsPackable)' == 'true' and '$(PackageId)' == ''">$(MSBuildProjectName)</PackageId>
+
+    <!-- For compatiblity with SDK Pack, we set a default description too -->
+    <Description Condition="'$(Description)' == ''">Package Description</Description>
   </PropertyGroup>
 
   <ItemDefinitionGroup>