do not always switch the mount being rendered in the channel mgr in case another mount is hit

When navigating through links on a website in the channel manager, if can happen that a link leads to a different channel (that might even belong to a different webapp).

When a different channel is opened, on the cms session context we set the attribute that indicates what is the currently being rendered mount (channel) in the channel manager.

However, this action of setting a different channel as the currently being rendered mount also happens for a normal hst request...on any webapp. As a result, it might happen that for example a platform mount is requested to process some rest request. In this case, the channel mgr should not 'think' a different mount is rendered in the channel mgr.

Typically, mounts that are never rendered in the channel mgr do not have a channel. Hence, we should at least skip resetting the rendering mount in case a request is processed by a mount that does not have a channel