Loader is never shown if hover provider is called before providers are registered

Created by: lguychard

The hover tooltip has been designed to show a loader if no provider has emitted after 1200ms. However we have noticed some cases where this didn't work, for instance https://github.com/sourcegraph/sourcegraph/issues/9346#issuecomment-604581293:

While the message hasn't gotten a response yet, we don't show a loader. I don't know why, I'd assume this is a webapp bug we need to look into. We should show a loader in the hover after a very short time.

I managed to reproduce this locally. This will always happen when hovering over the token before extensions have activated:

• Since no providers are registered, getLocationsFromProviders() will emit an empty array
• As a result, getDefinitionUrl() will emit null
• As a result, definitionUrlOrError will emit before LOADER_DELAY has lapsed, causing the loader to never show