You can try using a $match before the $lookup, to exclude the documents with the non-existing field:
{ $match: { someField: { $exists: true } } }
The alternative workaround to use the $ifNull to substitute a null for non-existing field is fine too. Substitute let: { someField : '$someField' } with let: { someField: { $ifNull: ["$someField", null ] } }
In applications it is not unusual to have unusual cases - data or logic. It just happens this case is a program shortcoming (and temporary only). It is not that bad to have a case by covering with an additional condition, if it helps running the program efficiently. Some documentation around this workaround helps for reference and apply the issue fix later on when available.