ToTemporalZonedDateTime: assertion failure with `{ offset: undefined }`

[Ms2ger]

1. If Type(item) is Object, then
   1. Let offsetString be ? Get(fields, "offset").
   2. If offsetString is not undefined, set it to ? ToString(offsetString).
2. Let offsetNanoseconds be ? ParseTimeZoneOffsetString(offsetString).

where ParseTimeZoneOffsetString starts

1. Assert: Type(offsetString) is String.

[ptomato]

This is fixed, I believe. If offsetString is undefined, we set offsetBehaviour to "wall", and we no longer go into ParseTimeZoneOffsetString.