Fix GET users with no users

ober.go

@@ -110,8 +110,12 @@ func UserGet(ctx *fasthttp.RequestCtx) {
 	} else {
 		regUsers, err := ReadRegisteredUsers()
 		if err != nil {
-			addErrorToCtx(ctx, err)
-			return
+			// check if no users registered
+			if err != nutsdb.ErrBucketEmpty {
+				addErrorToCtx(ctx, err)
+				return
+			}
+			regUsers = make([]UserData, 0)
 		}
 		for _, u := range regUsers {
 			user, _, err := GetUserData(u.Username)