`server:exec` doesn't support `is_async` - it just returns `nil` and nothing else

[Gerold103]

The reason is that it thinks it will call eval([[return pcall(...)]] internally and drops first returned value. But with is_async the first returned value is the future object. It shouldn't be dropped. Moreover, when is_async is used, it should use any pcall wrappers on the server. It should return the result as is.