For SERVER-63690 I'm implementing the basic path handling that does not match the current semantics of the classic engine (see SERVER-64060).

The problem here is to distinguish between the set of key values that was empty due to a path that didn't traverse any arrays but resulted in missing (in classical it matches to null), vs a path that traversed arrays and all terminals were missing (in classical it matches to null because an empty array matches to null), vs a path that traverses arrays and some terminals are missing and others aren't (in classical this does not match to null), vs all terminals being empty arrays (per SERVER-63368 classical will produce no matches in this case)

In SBE it's easy to implement either:
1. Treat any missing terminal as null and match records with such terminals to null/missing (this will match to null when some of the terminals are missing but not all)
2. For empty set of key values (because neither of the terminals produced a non-missing value) produce no matches (this won't match to null a path with no arrays that terminates in missing)
3. Match empty set of key values (because neither of the terminals produced a non-missing value) to null (this will match to null empty arrays).