Return an empty topology when no there is no known topology

[npepinpe]

Description

When the client makes a topology call to the gateway, if the gateway hasn't computed any topology, it currently returns UNAVAILABLE. However, when it has one (even if it's empty), it will return an empty topology.

For normal calls that require a partition (which is to say, most calls), when there is an empty topology or no known topology, we always return UNAVAILABLE. This makes perfect sense, since the gateway acts as a proxy, and in that sense upstream is not available.

However, the topology call is a way to introspect the view of the cluster from the gateway's point of view. Returning UNAVAILABLE hides that and makes it hard to distinguish between a call where the gateway is unavailable and where the topology is simply not known.

I would propose instead to simply return an empty topology. The smoke test in StandaloneGatewayIT would have to be updated to account for this, and possibly other tests.

[npepinpe]

Extracted from #9077