Embed metrics undefined param fix

What does this MR do?

Fixes a bug where a metrics dashboard clipboard link param could be undefined some of the time.

To reproduce:

1. Set the following feature flag:
   • environment_metrics_show_multiple_dashboards = false
2. Navigate to the metrics dashboard.
3. Open the dropdown next to a chart.
4. Select 'Generate link to chart' button.
5. Inspect the clipboard.

Before fix	After fix
http://localhost:3001/root/my-project/environments/1/metrics?group=Kubernetes&title=CPU%20usage&y_label=CPU&dashboard=undefined	http://localhost:3001/root/my-project/environments/1/metrics?group=Kubernetes&title=CPU%20usage&y_label=CPU

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66177

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
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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

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• 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
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