Prefer nil empty slices

Signed-off-by: David Gageot <david.gageot@docker.com>

pkg/mount/mountie.go

@@ -197,7 +197,7 @@ func makeDevLinks() error {
 
 // return a list of all available drives
 func findDrives() []string {
-	driveKeys := []string{}
+	var driveKeys []string
 	ignoreExp := regexp.MustCompile(`^loop.*$|^nbd.*$|^[a-z]+[0-9]+$`)
 	devs, _ := ioutil.ReadDir("/dev")
 	for _, d := range devs {