Fix showing Avg: NaN · Max: NaN in MR analytics

What does this MR do and why?

Updates the MR analytics charts to correctly handle empty / no data.

• Updates the single stat to display - when there is no data and to use numeric values instead of strings
• Updates the avg / max calculations to avoid NaN appearing

Screenshots or screen recordings

Before - No data	After - No data	Before - Data	After - Data

How to set up and validate locally

1. Create a new project
2. Navigate to merge request analytics
3. Create some issues / MRs
4. Return to the chart

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 #357633 (closed)