ResxSourceGenerator.CSharp generates invalid code when RootNamespace is empty

I'm using Microsoft.CodeAnalysis.ResxSourceGenerator, in a test project for which `RootNamespace` is set to be empty.
My .resx file is in a subdirectory. The source generated code-behind starts with:

```cs
namespace .Mocks
{
    internal static partial class MockResources
    {
```

Note the `.Mocks` namespace, as if a non-empty RootNamespace was an assumed invariant.