Add defensive null guards to all chart components and analytics

All 9 chart components now have early-return null/empty checks before
calling .map() on data props. The diversity-metrics chart guards all
nested array fields (byCountry, byCategory, byOceanIssue, byTag).
Analytics backend guards p.tags in getDiversityMetrics. This prevents
any "Cannot read properties of null (reading 'map')" crashes even if
upstream data shapes are unexpected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/charts/juror-workload.tsx

@@ -25,6 +25,8 @@ type WorkloadBarDatum = {
 }
 
 export function JurorWorkloadChart({ data }: JurorWorkloadProps) {
+  if (!data?.length) return null
+
   const totalAssigned = data.reduce((sum, d) => sum + d.assigned, 0)
   const totalCompleted = data.reduce((sum, d) => sum + d.completed, 0)
   const overallRate =