Fix panic when closing nats connection

[gmgigi96]

Fixes this panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x10d6188]

goroutine 1 [running]:
github.com/nats-io/nats%2ego.(*Conn).Drain(0x0)
        /home/gianmaria/go/pkg/mod/github.com/nats-io/[email protected]/nats.go:5190 +0x28
github.com/cs3org/reva/pkg/notification/notificationhelper.(*NotificationHelper).Stop(0xc000282f00)
        /home/gianmaria/Documenti/CERN/reva2/pkg/notification/notificationhelper/notificationhelper.go:124 +0x29
github.com/cs3org/reva/internal/http/services/owncloud/ocdav.(*svc).Close(0xc0007ca300)
        /home/gianmaria/Documenti/CERN/reva2/internal/http/services/owncloud/ocdav/ocdav.go:188 +0x34
github.com/cs3org/reva/pkg/rhttp.(*Server).closeServices(0xc0002180e0)
        /home/gianmaria/Documenti/CERN/reva2/pkg/rhttp/rhttp.go:154 +0xb7
github.com/cs3org/reva/pkg/rhttp.(*Server).Stop(0xc0002180e0)
        /home/gianmaria/Documenti/CERN/reva2/pkg/rhttp/rhttp.go:142 +0x45
github.com/cs3org/reva/cmd/revad/pkg/grace.(*Watcher).TrapSignals(0xc00098b790)
        /home/gianmaria/Documenti/CERN/reva2/cmd/revad/pkg/grace/grace.go:475 +0x9fd
github.com/cs3org/reva/cmd/revad/runtime.(*Reva).Start(0xc000368140)
        /home/gianmaria/Documenti/CERN/reva2/cmd/revad/runtime/runtime.go:238 +0x358
main.runSingle(0xc000719e80, {0xc00003f900, 0x33})
        /home/gianmaria/Documenti/CERN/reva2/cmd/revad/main.go:238 +0x266
main.runConfigs({0xc0005364a0, 0x1, 0x1})
        /home/gianmaria/Documenti/CERN/reva2/cmd/revad/main.go:217 +0x70
main.main()
        /home/gianmaria/Documenti/CERN/reva2/cmd/revad/main.go:89 +0x491

This was happening when the connection to nats server was not yet established, and a close to the notification service was triggered.