Keep builder state when a property is None

[pojntfx]

With the current state on main, if you pass in a property as None, the whole builder gets overwritten. This PR changes that so in the case where a property is None, we just return the builder unchanged.

This fixes connecting to a bootstrap node - with the state on main, if you disable mDNS and connect via a bootstrap node, nothing ever connects otherwise since every None argument would wipe all preceding arguments, e.g. the bootstrap URL.

Heads up: My Rust is very rusty, so there might be a better way to do this I'm not aware of?

[adzialocha]

I'm not a glib expert but I think this looks good!

From a Rust perspective you could do an early return for value right at the beginning before you go into the match statement, something like that:

let Some(value) = value else {
    return builder
};

Now you can assume that value will always be Some for all code coming after.