insights: dashboard pings can report error if no insights are associated to a dashboard

Created by: coury-clark

We have a dashboard ping that returns basic statistics about the number of insights on dashboards. This is failing to account for nil situations, where there aren't any insights on dashboards.

ERROR An error occurred in a background task, handler: insights_pings_emitter, error: emitInsightsPerDashboard: GetInsightsPerDashboard: sql: Scan error on column index 3, name "stddev": converting NULL to float32 is unsupported