bugfix: skip ref validation when Sidecar(ref=None)

[bmorris3]

On initialization, Sidecar(ref=reference_widget) validates the reference widget's anchor, to ensure that a anchor='right' widget can't be used as a reference.

Before this PR, if a user explicitly set ref=None in the constructor, ref has no anchor attribute, and the validation raises an error.

This PR checks for initialization with ref is None, and skips validation if so.

Demo example:

from sidecar import Sidecar
from ipywidgets import widgets

with Sidecar(anchor='split-bottom', title='a', ref=None):
    display(widgets.Button())

The demo above doesn't work before this PR, and does work after this PR.

[martinRenou]

Thanks!