If not namespace is provided, don't create double slashes

[joehoyle]

In cases where I'm writing a full bespoke/whitelabelled API rather than adding to WordPress's routes, or them even being available, I want to not use namespaces at all.

This is probably controversial - but point being I want my api at myserviers.com/api for better or worse. (In this specific case I'm migrating from another API system, so I need the URLs to match the old ones as it's a drop in replacement).

In this case, it's pretty easy for us to support this by just checking if a namespace was not-empty and don't add the double slashes if so.

[rmccue]

I'd probably rather do a _doing_it_wrong, butttt I dunno.

[rachelbaker]

I feel like this allows registered routes to skirt around namespacing their routes too easily, which is something we want to avoid. I agree with @rmccue that we should probably display a debug message.

[rmccue]

Thought: _doing_it_wrong in register_rest_route, but allow it inside WP_REST_Server::register_route.

[joehoyle]

Given the incorrect usage capture here, I'm ok with the tests at 60% given calling it will cause a notice anyway and fail the tests!