feat: tieredProxyUrls accept null for switching the proxy off

[barjin]

Since #2392, the ProxyConfiguration class is ready for null proxy "URLs" - this means that the proxy won't be used.

This already worked with newUrlFunction, but because of types and validation, users couldn't use it with tieredProxyUrls.

This PR relaxes the type checks and validation so we can use stuff like:

new ProxyConfiguration({
    tieredProxyUrls: [
        [null], 
        ['http://proxy.apify.com', 'http://some.other.proxy.com']
    ],
})

and even

new ProxyConfiguration({
    tieredProxyUrls: [
-       [null], 
+       [null, 'http://some.local.cheap.proxy', 'http://other.local.cheap.proxy'], 
        ['http://proxy.apify.com', 'http://some.other.proxy.com']
    ],
})

Closes #2740

[janbuchar]

Seems OK! Could you make an issue for porting this to crawlee-python please?

[barjin]

Here you go:

apify/crawlee-python#687