[IndexedDB] Attributes with undefined vs. null

Various atttributes in IndexedDB signal "no value" with |undefined|:

IDBKeyRange.lowerBound (if not set)
IDBKeyRange.upperBound (if not set)
IDBRequest.result (on error, or on successful deleteDatabase/get with no
value/delete/clear)
IDBCursor.key (if no found record)
IDBCursor.primaryKey (if no found record)
IDBCursorWithValue.value (if no found record)

It's been pointed out that most Web platform specs use |null| rather than
|undefined| for signaling these states. I seem to recall a push in the
direction of using |undefined| rather than |null| in the IndexedDB spec bit
over a year ago, but my bugzilla-fu was weak. Can anyone discuss or justify
this deviation from the norm?

(I feel like there's been a trend over the past few years in embrace
ECMAScript's |undefined| value rather than trying to pretend it doesn't
exist, but that may be my imagination. IDB's use of |undefined| didn't
strike me as unusual until it was pointed out.)