Fix MR throughput on the Value Stream Dashboard

What does this MR do and why?

Send null instead of [] when the labels filter is blank, otherwise no data will be returned for a blank filter.

Screenshots or screen recordings

Before
After

How to set up and validate locally

1. Feature.enable(:combined_analytics_dashboards)
2. Feature.enable(:vsd_graphql_dora_and_flow_metrics)
3. Setup YAML configuration
4. Seed analytics data (and optionally DORA metrics data)
5. Visit the value streams dashboard: http://gdk.test:3000/groups/${YOUR_GROUP}/-/analytics/dashboards
6. Create and merge an MR for the test group
7. Validate that a VSD correctly loads the MR throughput

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #418957 (closed)