Editorial: Fix type assertion in TotalRelativeDuration

NudgeToCalendarUnit accepts a sign of -1 or 1, not 0, but it was
possible for 0 to be passed in TotalRelativeDuration, which would fail a
type assertion in the spec. However, 0 can just be treated as 1 for the
purposes of NudgeToCalendarUnit. We already do the same thing in
RoundRelativeDuration.

(An alternative would be to add a fast path when the sign is 0, to just
return a total of 0 directly. This would also be an unobservable
editorial change.)

Closes: #3131

Author: ptomato

spec/duration.html

@@ -1853,7 +1853,7 @@ <h1>
       </dl>
       <emu-alg>
         1. If IsCalendarUnit(_unit_) is *true*, or _timeZone_ is not ~unset~ and _unit_ is ~day~, then
-          1. Let _sign_ be InternalDurationSign(_duration_).
+          1. If InternalDurationSign(_duration_) < 0, let _sign_ be -1; else let _sign_ be 1.
           1. Let _record_ be ? NudgeToCalendarUnit(_sign_, _duration_, _destEpochNs_, _isoDateTime_, _timeZone_, _calendar_, 1, _unit_, ~trunc~).
           1. Return _record_.[[Total]].
         1. Let _timeDuration_ be ! Add24HourDaysToTimeDuration(_duration_.[[Time]], _duration_.[[Date]].[[Days]]).