Expose dates where DORA data is missing

What does this MR do and why?

This MR updates the DORA charts within the Insights feature to expose dates with nil value where metrics were not recorded.

The problem is described with a screenshot in the issue: #371885 (closed)

The BE change will make a gap in the chart:

How to set up and validate locally

You can generate DORA data using the following instructions: !93544 (merged)

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 #371885 (closed)