insights: fix panic caused by missing base resolver (and nil store)

Created by: coury-clark

Fixes https://github.com/sourcegraph/sourcegraph/issues/27245

There was a resolver (insightsDashboardResolver) that didn't drill down the baseInsightResolver, and therefore had a nil insightStore.

Here is a query that should capture all of these, to verify we have them all now. https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+file:insights/resolvers/.*+count:all+Resolver%7B...%7D%2C+nil&patternType=structural