zenno: allow both stats in threshold modifier to be null

zenno/src/lib/race.ts

@@ -275,7 +275,13 @@ export function adjustedStat(
 	}
 }
 
-export function thresholdMod(stat1: number, stat2?: number): number {
+export function thresholdMod(stat1: number | null | undefined, stat2?: number | null): number {
+	if (stat1 == null) {
+		if (stat2 == null) {
+			return 1;
+		}
+		return thresholdMod(stat2);
+	}
 	if (stat2 != null) {
 		return 0.5 * (thresholdMod(stat1) + thresholdMod(stat2));
 	}