Resolve "Productivity Analytics: MR table raises a JS error occasionally"

What does this MR do?

This MR fixes a bug on the Productivity Analytics page when we receive null values for certain attributes of an MR, such as commits_count.

The reason for this is we pass the commit_count directly to n__ for proper pluralization. However, n__ apparently doesn't accept null values which leads to this issue.

This MR introduces a sanity check and makes sure to avoid null values from being passed to n__. Instead, we won't render the metric at all.

This is still behind the productivity_analytics feature flag.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #32343 (closed)