Snaps CLI should warn or error if “endowment:rpc” object is empty

[Montoya]

If a Snap manifest has the RPC endowment with an empty object like so:

“endowment:rpc”: {}

This will cause an error in MetaMask when attempting to connect like so: An error happened: Invalid JSON-RPC origins: Must specify at least one JSON-RPC origin

The CLI should warn that the endowment will not be functional without at least one origin such as "dapps":true

[FrederikBolding]

This happens because Superstruct assign doesn't copy over refinements, so when merging two structs here we lose some validation.