Improve DryIoc error message when factory delegate returns null

It seems like this exception is missing details -- and it'd be great if we could even catch this and log something more directly applicable to the user:

```
DryIoc.ContainerException : Registered factory delegate returns service is not assignable to ambiently scoped container with {no name, Parent={no name}}.
```

You can repro this if you do something like this in your Startup.cs:

```
public override void Configure(IFunctionsHostBuilder builder)
{
builder.Services.AddSingleton(s =>
{
return null;
});
}
```

You wouldn't explicitly do this -- but some factory delegates can potentially return null, and when they do you are left with a fairly confusing message.

Start with the Startup.cs registration shown in the issue and reproduce the ContainerException when the factory delegate returns null. Trace where this exception message is produced and verify that the completed change gives users more actionable details for a null factory result.

Written by the indexing model from the issue text.