Constant Nothing in $group's _id expression should be converted to null in $group SBE stage builder

> db.a.insert({_id: 1});

> db.a.explain().aggregate([{$group: {_id: "$$REMOVE", o: {$first: "$a"}}}]).queryPlanner.winningPlan.slotBasedPlan.stages;

[2] mkbson s9 [_id = s6, o = s8] true false

[2] group [s6] [s8 = first (fillEmpty (s7, null))]

[2] project [s7 = getField (s4, "a")]

[2] project [s6 = Nothing]

[1] scan s4 s5 none none none none [] @"39336b7b-b5b8-4328-8ae6-e5a7e9ce31ae" true false