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/cross-round-comparison.tsx

@@ -21,6 +21,16 @@ interface CrossStageComparisonProps {
 export function CrossStageComparisonChart({
   data,
 }: CrossStageComparisonProps) {
+  if (!data?.length) {
+    return (
+      <Card>
+        <CardContent className="flex items-center justify-center py-12">
+          <p className="text-muted-foreground">No comparison data available</p>
+        </CardContent>
+      </Card>
+    )
+  }
+
   const baseData = data.map((round) => ({
     name:
       round.roundName.length > 20