fix(org-stats): Prevent TypeError when group.totals or group.series is undefined

[sentry]

This PR addresses a TypeError: Cannot read properties of undefined (reading 'sum(quantity)') (JAVASCRIPT-3B74) that occurred in static/app/views/organizationStats/usageStatsProjects.tsx and static/app/views/organizationStats/mapSeriesToChart.ts.

The root cause was identified as the application attempting to access group.totals['sum(quantity)'] or group.series['sum(quantity)'] without proper null or undefined checks. The stats_v2 API can return group entries where the totals or series fields are missing (e.g., when there is no data for a specific outcome, category, or project combination).

The fix involves adding optional chaining (?.) and nullish coalescing (?? 0 or ?? []) to safely access these properties, ensuring that the application gracefully handles cases where group.totals or group.series might be undefined.

During the CI process, a formatting issue was identified by the pre-commit hook. This was resolved by extracting a potentially long inline expression into a separate variable to improve readability and adhere to the project's formatting standards.

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Fixes JAVASCRIPT-3B74

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}