Previously we did not allow missing fields in documents for PBT tests, because covered queries do not distinguish null/missing. There are other ways to test missing:

• Allow null and missing, but don't create indexes on the collection
• Allow null and missing, but skip any queries that use index-only plans
• Allow missing but not null (indexes are allowed). The documents can take any shape this way (can use fc.object() for example)

The third property would have caught SERVER-110423. There may be other missing bugs