Add error handling in case the paths for gNMI Subscription for an MNE is empty

Description

Currently, it doesn't matter if there are paths or if the value for the Path in gNMI Subscribe Option is nil. The latter results in a device throwing an error that it's not possible to subscribe because of no provided paths. A user/client doesn't get this information though. Instead, an error should be forwarded to the calling client. This could be done in networkElementWatcher.go/subscribeToNetworkElement (somewhere around line 100 where the paths are merged)