Schema queries do not return predicates

[matthewmcneely]

Issuing a schema {} query when in embedded/file mode does not return the predicates from the group. Seems to be some issue in worker/schema.go in that the "known groups" in modusGraph are nil.

In regular Dgraph, even with an empty schema request, the addToSchemaMap function is adding group 1 to the map through the groups().KnownGroups() call. But in embedded mode, that function is returning an empty list.

To Reproduce

Issue a DQL query of schema{}. Only the types are being returned.

[linear]

HYP-3312 Schema queries do not return predicates