strtictNullChecks in PT, TZ, and ZDT code

This commit makes a handful of type changes in plaintime.ts,
timezone.ts, and zoneddatetime.ts to handle parameters or
variables that may be undefined or null. There's only a few
changes.

Author: justingrant

lib/plaintime.ts

@@ -43,7 +43,7 @@ type TemporalTimeToStringOptions = {
 function TemporalTimeToString(
   time: Temporal.PlainTime,
   precision: ReturnType<typeof ES.ToSecondsStringPrecision>['precision'],
-  options: TemporalTimeToStringOptions = undefined
+  options: TemporalTimeToStringOptions | undefined = undefined
 ) {
   let hour = GetSlot(time, ISO_HOUR);
   let minute = GetSlot(time, ISO_MINUTE);

lib/timezone.ts

@@ -125,7 +125,7 @@ export class TimeZone implements Temporal.TimeZone {
       return null;
     }
 
-    let epochNanoseconds = GetSlot(startingPoint, EPOCHNANOSECONDS);
+    let epochNanoseconds: JSBI | null = GetSlot(startingPoint, EPOCHNANOSECONDS);
     const Instant = GetIntrinsic('%Temporal.Instant%');
     epochNanoseconds = ES.GetIANATimeZoneNextTransition(epochNanoseconds, id);
     return epochNanoseconds === null ? null : new Instant(epochNanoseconds);
@@ -140,7 +140,7 @@ export class TimeZone implements Temporal.TimeZone {
       return null;
     }
 
-    let epochNanoseconds = GetSlot(startingPoint, EPOCHNANOSECONDS);
+    let epochNanoseconds: JSBI | null = GetSlot(startingPoint, EPOCHNANOSECONDS);
     const Instant = GetIntrinsic('%Temporal.Instant%');
     epochNanoseconds = ES.GetIANATimeZonePreviousTransition(epochNanoseconds, id);
     return epochNanoseconds === null ? null : new Instant(epochNanoseconds);

lib/zoneddatetime.ts

@@ -225,9 +225,9 @@ export class ZonedDateTime implements Temporal.ZonedDateTime {
         entries.push([fieldName, undefined]);
       }
     });
-    let fields = ES.PrepareTemporalFields(this, entries as any);
+    let fields = ES.PrepareTemporalFields(this, entries);
     fields = ES.CalendarMergeFields(calendar, fields, props);
-    fields = ES.PrepareTemporalFields(fields, entries as any);
+    fields = ES.PrepareTemporalFields(fields, entries);
     const { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } =
       ES.InterpretTemporalDateTimeFields(calendar, fields, options);
     const offsetNs = ES.ParseTimeZoneOffsetString(fields.offset);