insights: landing page displays sql error if no insight views exist

Created by: leonore

On a fresh local sg instance (sg start enterprise-codeinsights), the /insights landing page errors with ERROR: syntax error at or near ")" (SQLSTATE 42601) if no insight views exist. This can be fixed by first navigating to /insights/create and creating an insight.

This error seems to come from https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/enterprise/internal/insights/store/insight_store.go?L134, where we format a SELECT query based on an id IN (%s) where the query is executed even if there are no IDs to find views for.