The VersionContext type can represent either an operation with an associated Operation FCV, or an operation without OFCV. Currently, the "Operation without OFCV" state can be serialized/deserialized to an empty BSON object. This possibility is ultimately unused, since a VersionContext-typed field can be made optional in IDL and makes for a cleaner and more efficient representation.

However this lingering serialization capability is a source of bugs. The following two commands are semantically identical but have different internal representations (empty optional vs Operation without FCV) so they could behave differently:

db.runCommand({create: "x1"})
db.runCommand({create: "x1", versionContext: {}})

Remove the possibility to serialize or deserialize a VersionContext without OFCV in order to forbid the later case.