Make TS happier with year/eraYear

This was originally part of tc39/proposal-temporal#1977
but I removed it because it wasn't high-priority for JS.
But it's still needed in this repo so I'm adding it back.

Why? TS complains when you assign a maybe-undefined
value to a must-not-be-undefined variable. This small
runtime change is cleaner than adding a lotta type casts.

Author: justingrant

lib/calendar.ts

@@ -1305,8 +1305,8 @@ const helperHebrew: NonIsoHelperBase = ObjectAssign({}, nonIsoHelperBase, {
     fromLegacyDate = false
   ) {
     let { year, eraYear, month, monthCode, day, monthExtra } = calendarDate;
-    if (year === undefined) year = eraYear;
-    if (eraYear === undefined) eraYear = year;
+    if (year === undefined && eraYear !== undefined) year = eraYear;
+    if (eraYear === undefined && year !== undefined) eraYear = year;
     if (fromLegacyDate) {
       // In Pre Node-14 V8, DateTimeFormat.formatToParts `month: 'numeric'`
       // output returns the numeric equivalent of `month` as a string, meaning