fix(monitors): Prevent crash when cron detector queryObj is null

[sentry]

This PR addresses a TypeError: Cannot read properties of null (reading 'slug') occurring in CronDetectorDetails when dataSource.queryObj is null.

The root cause was that the frontend code assumed dataSource.queryObj would always be present, but the API can return a valid CronDetector where this property is null (e.g., for misconfigured or partially-created monitors).

The solution implements the following changes:

1. CronDetectorDetails (static/app/views/detectors/components/details/cron/index.tsx):

   • Introduced an early return null guard when dataSource.queryObj is null, ensuring that subsequent code relying on queryObj is not executed.
   • Moved useState and useCallback hooks to before the early return to comply with React's Rules of Hooks.
   • Updated calls to useDocsPanel and useMonitorProcessingErrors to use optional chaining (?.) for dataSource.queryObj.slug.
   • Modified getLatestCronMonitorEnv to safely handle a null queryObj.

2. useMonitorProcessingErrors (static/app/views/insights/crons/useMonitorProcessingErrors.tsx):

   • Widened the monitorSlug type to string | undefined.
   • Added an enabled: !!monitorSlug option to useApiQuery to prevent API calls when monitorSlug is undefined.
   • Ensured internal calls to deleteMonitorProcessingErrorByType provide a fallback empty string (monitorSlug ?? '') when monitorSlug is undefined.

3. useDocsPanel (static/app/views/detectors/components/details/cron/index.tsx):

   • Widened the monitorSlug parameter type to string | undefined.
   • Passed monitorSlug ?? '' to MonitorQuickStartGuide to satisfy its monitorSlug: string prop type.

Fixes JAVASCRIPT-3BHN

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