Consider default private upstream servers

The syntax of upstream servers allows using `#` within the domain-specific server to make AdGuard Home use the default one for specified domains. Currently, it requires at least a single non-domain-specific server to be presented.

I suppose we may skip this step of validation for "Private reverse DNS servers" and use the default ones if those are already determined automatically and all the entries are domain-specific. @ameshkov, what do you think?