Fix missing prop warning

What does this MR do and why?

Updates the ee/spec/frontend/usage_quotas/product_analytics/components/group_usage/product_analytics_group_monthly_usage_chart_spec.js to add a default isLoading=false in the component creation method to fix a warning for a missing prop.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual code changes, only fixes a terminal warning when running tests.

Before	After

How to set up and validate locally

Run the test locally

$ yarn jest --watch ee/spec/frontend/usage_quotas/product_analytics/components/group_usage/product_analytics_group_monthly_usage_chart_spec.js