insights: fix sql error on landing page when no insights exist

Created by: leonore

closes #32510 (closed)

Test plan

Ran insight store unit test. Compared behaviour of insights landing page before/after change, with a sg instance launched using sg start enterprise-codeinsights

Prior to the change, insights/ and /insights/dashboards would display an SQL error. After the change, the landing page is correct and invites you to create an insight.