[FIX] web_editor: prevent creation of RTCPeerConnection when offline

Before this commit, when the browser was offline and when an attempt
to make a RTCPeerConnection was made, a traceback was raised by
firefox:

> InvalidStateError: Can't create RTCPeerConnections when the network is
> down

This commit prevents the creation of the RTCPeerConnection when the
browser is offline.

task-3186872

closes odoo/odoo#133770

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>