Minor type change to ecmascript.ts per @ptomato

Author: justingrant

lib/ecmascript.ts

@@ -3332,7 +3332,7 @@ export function UnbalanceDurationRelative(
   const sign = DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
 
   let calendar;
-  let relativeTo = undefined as Temporal.PlainDate | undefined;
+  let relativeTo: Temporal.PlainDate | undefined;
   if (relativeToParam) {
     relativeTo = ToTemporalDate(relativeToParam);
     calendar = GetSlot(relativeTo, CALENDAR);
@@ -3434,7 +3434,7 @@ export function BalanceDurationRelative(
   if (sign === 0) return { years, months, weeks, days };
 
   let calendar;
-  let relativeTo = undefined as Temporal.PlainDate | undefined;
+  let relativeTo: Temporal.PlainDate | undefined;
   if (relativeToParam) {
     relativeTo = ToTemporalDate(relativeToParam);
     calendar = GetSlot(relativeTo, CALENDAR);