Ensure ObjectValue.ToStringValue() doesn’t return null

[JanRomero]

Since ToString() can return null, we must guard against this to prevent an ArgumentNullException later in WriteToAsync() when the string value gets encoded.

[sebastienros]

Can you provide an example that would trigger such exception in a template?

[JanRomero]

@sebastienros sure, this may be contrived but this’ll do it:

public record struct MyModel(string Title, string? Value)
{
    public override string? ToString() => Value;
}

var template = fluidParser.Parse("""
        Title: {{MyItem.Title}}
        Value: {{MyItem}}        
        """);

var dict = new Dictionary<string, object>() { ["MyItem"] = new MyModel() };

template.Render(new TemplateContext(dict)); //System.ArgumentNullException: "Value cannot be null. Arg_ParamName_Name"

Visual Studio will actually generate string? ToString() if nullable types are enabled for the project.

[sebastienros]

I decided to accept null values but handle them, there might be other places that already return these.

#842