TSLEncoder: Fix Fn layout generation when generating a function with no input parameters

[Ne0nWinds]

Description

When generating a function that doesn't have any input parameters, the TSLEncoder emits an extraneous comma, causing the generated JavaScript to be invalid.

This PR fixes that by only emitting a comma if there is at least one input parameter to the function.

Example
Creating a function with no input parameters like this:

float example() {
	return 1.0;
}

Causes the TSLEncoder to emit the following:

import { Fn } from 'three/tsl';

export const example = /*@__PURE__*/ Fn( () => {

	return 1.0;

}, { , return: 'float' } );

The second parameter to Fn (i.e, the function layout) has an invalidly placed comma before the return property.