tieredProxyUrls accept null for switching the proxy off

[barjin]

This is a parity-tracking issue for Crawlee JS / Crawlee Python.

With apify/crawlee#2743 , we enabled ProxyConfiguration.tieredProxyUrls to accept null as a command to switch the proxy off on that tier. In Crawlee JS, this was only a matter of widening the types and making the validation less strict - ProxyConfiguration was already prepared for null URLs (returning null from newUrlFunction - this seems to work in Crawlee for Python too).