Don't hide namespaces with history but no contents?

[mitchellwrosen]

Sometimes we hide namespaces with a history but no contents at the latest causal step in a few places (e.g. in ls, and when serving various payloads to the frontend).

But this causes confusion sometimes. In #3648, a user thinks they're pulling a namespace that doesn't exist, and they should get an error message, but they're simply pulling a namespace that we've filtered out in the UI.