[core] Exclude undefined className from render fn props

[mj12albert]

Fixes #993

When className is unspecified on the component, className inside the render function should not be overwritten by the renderFn's props.className:

<Toggle
  aria-label="Favorite"
  render={(props, state) => {
    if (state.pressed) {
      return (
        <button type="button" className={styles.Button} {...props}>
          <HeartFilledIcon />
        </button>
      );
    }

    return (
      <button type="button" className={styles.Button} {...props}>
        <HeartOutlineIcon />
      </button>
    );
  }}
/>

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[atomiks]

Test would be nice

[mj12albert]

Test would be nice

Added ~