fix(dgraph): Panic because of nil map in groups.go (dgraph-io#5999)

* Initialize delta.GroupChecksums if nil

delta.GroupChecksums is nil at this point in the code, so copying keys to this map causes the code to panic

* Address review comments

Remove if nil check

* fix CI; move map make to for loop since delta is assigned a struct from the deltaCh

worker/groups.go

@@ -953,6 +953,9 @@ func (g *groupi) processOracleDeltaStream() {
 						return
 					}
 					batch++
+					if delta.GroupChecksums == nil {
+						delta.GroupChecksums = make(map[uint32]uint64)
+					}
 					delta.Txns = append(delta.Txns, more.Txns...)
 					delta.MaxAssigned = x.Max(delta.MaxAssigned, more.MaxAssigned)
 					for gid, checksum := range more.GroupChecksums {