don't replace no data with 0 for budget graph

compendium-frontend/src/helpers/dataconversion.tsx

@@ -142,7 +142,7 @@ export function createBudgetDataset(budgetEntries: Budget[]): BasicDataset {
         return {
             backgroundColor: randomColor,
             borderColor: randomColor,
-            data: labelsYear.map((year) => dataLookup.get(nren)!.get(year)?.budget ?? 0),
+            data: labelsYear.map((year) => dataLookup.get(nren)!.get(year)?.budget ?? null),
             label: nren,
             hidden: false
         }