Treat empty json string in tool call as having 'no' arguments.

[mstancombe]

If a non-server tool has no arguments, it's currently still run through the json deserializer, which raises a JsonException.

This doesn't stop the chat client from working, CreateFromParsedArguments just seems to log the exception in the created FunctionCallContent, but it does mean a noisy exception, and the created FunctionCallContent doesn't reflect the true state of play.

This PR handles an empty string as meaning there was no arguments given, which I think is how we should handle this.

[sd-st]

Would you be able to rebase this?

[sd-st]

@stephentoub Does null make sense here or should re be returning an empty dictionary?

[mstancombe]

Just to expand on my reasoning, as I did look into this a bit before deciding on null, the 'Arguments' on FunctionCallContext class in MEAI looks like this:

/// <summary>
/// Gets or sets the arguments requested to be provided to the function.
/// </summary>
public IDictionary<string, object?>? Arguments { get; set; }

Which doesn't explicitly state when this should be null, but given that it is nullable, I think an llm passing no arguments probably should be interpreted as nothing.

Additionally, the debug view for FunctionCallContext does this:

/// <summary>Gets a string representing this instance to display in the debugger.</summary>
[DebuggerBrowsable(DebuggerBrowsableState.Never)]
private string DebuggerDisplay
{
    get
    {
        string display = "FunctionCall = ";

        if (CallId is not null)
        {
            display += $"{CallId}, ";
        }

        display += Arguments is not null ?
            $"{Name}({string.Join(", ", Arguments)})" :
            $"{Name}()";

        return display;
    }
}

If arguments was an empty dict, this would show as 'MyTool()' vs 'MyTool' when it's null. Again, this doesn't prove what the intention was, but I'm guessing (hoping :P) that @stephentoub was expecting a null to represent 'no args'.

[mstancombe]

ah, disregard the final point, it would still show MyTool() with no args.. it just would skip the string.join over the empty dictionary, but no output difference.

[mstancombe]

Rebased to latest next.