Fix nonconformance bug with ZonedDateTime.withPlainTime

Previously, zdt.withPlainTime(null) would default to midnight. According
to the spec text, it should throw a TypeError.

Test262 tests for this are in review.

UPSTREAM_COMMIT=07729749f08c4a5c89af283f7a059fb69e37bd41

Author: ptomato

lib/zoneddatetime.ts

@@ -290,7 +290,7 @@ export class ZonedDateTime implements Temporal.ZonedDateTime {
     if (!ES.IsTemporalZonedDateTime(this)) throw new TypeError('invalid receiver');
 
     const PlainTime = GetIntrinsic('%Temporal.PlainTime%');
-    const temporalTime = temporalTimeParam == undefined ? new PlainTime() : ES.ToTemporalTime(temporalTimeParam);
+    const temporalTime = temporalTimeParam === undefined ? new PlainTime() : ES.ToTemporalTime(temporalTimeParam);
 
     const thisDt = dateTime(this);
     const year = GetSlot(thisDt, ISO_YEAR);