Editorial: Return zero early in BalancePossiblyInfinite...Relative

This avoids calling NanosecondsToDays with the precalculated datetime
parameter present but undefined, which is disallowed according to that
operation's type assertions.

Credits to Anba for catching this.

Closes: #2694

polyfill/lib/ecmascript.mjs

@@ -3552,9 +3552,12 @@ export function BalancePossiblyInfiniteTimeDurationRelative(
 
   const endNs = AddInstant(intermediateNs, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
   nanoseconds = endNs.subtract(startNs);
+  if (nanoseconds.isZero()) {
+    return { days: 0, hours: 0, minutes: 0, seconds: 0, milliseconds: 0, microseconds: 0, nanoseconds: 0 };
+  }
 
   if (largestUnit === 'year' || largestUnit === 'month' || largestUnit === 'week' || largestUnit === 'day') {
-    if (!nanoseconds.isZero()) precalculatedPlainDateTime ??= GetPlainDateTimeFor(timeZone, startInstant, 'iso8601');
+    precalculatedPlainDateTime ??= GetPlainDateTimeFor(timeZone, startInstant, 'iso8601');
     ({ days, nanoseconds } = NanosecondsToDays(nanoseconds, zonedRelativeTo, precalculatedPlainDateTime));
     largestUnit = 'hour';
   } else {

spec/duration.html

@@ -1351,8 +1351,9 @@ <h1>
           1. Set _intermediateNs_ to _intermediateResult_.[[EpochNanoseconds]].
         1. Let _endNs_ be AddInstant(_intermediateNs_, _hours_, _minutes_, _seconds_, _milliseconds_, _microseconds_, _nanoseconds_).
         1. Set _nanoseconds_ to ℝ(_endNs_ - _zonedRelativeTo_.[[Nanoseconds]]).
+        1. If _nanoseconds_ = 0, return ! CreateTimeDurationRecord(0, 0, 0, 0, 0, 0, 0).
         1. If _largestUnit_ is one of *"year"*, *"month"*, *"week"*, or *"day"*, then
-          1. If _nanoseconds_ &ne; 0 and _precalculatedPlainDateTime_ is *undefined*, set _precalculatedPlainDateTime_ to ? GetPlainDateTimeFor(_zonedRelativeTo_.[[TimeZone]], _startInstant_, *"iso8601"*).
+          1. If _precalculatedPlainDateTime_ is *undefined*, set _precalculatedPlainDateTime_ to ? GetPlainDateTimeFor(_zonedRelativeTo_.[[TimeZone]], _startInstant_, *"iso8601"*).
           1. Let _result_ be ? NanosecondsToDays(_nanoseconds_, _zonedRelativeTo_, _precalculatedPlainDateTime_).
           1. Set _days_ to _result_.[[Days]].
           1. Set _largestUnit_ to *"hour"*.