dbHash doesn't run on clustered collections

As clustered collections do not have an _id index, dbHash fails silently. dbHash will only opt to perform a collection scan on capped collections instead of an index scan on _id.

https://github.com/mongodb/mongo/blob/297904e17aaa5351d729685b70b97027e44f0cf9/src/mongo/db/commands/dbhash.cpp#L346-L368