Return empty slices instead of nil for ListAllUsers

internal/server/usermanager/localmanager.go

@@ -191,6 +191,9 @@ func (manager *localManager) ListAllUsers() (infos []UserInfo, err error) {
 		})
 		return err
 	})
+	if infos == nil {
+		infos = []UserInfo{}
+	}
 	return
 }
 

internal/server/usermanager/voidmanager.go

@@ -15,7 +15,7 @@ func (v *Voidmanager) UploadStatus(updates []StatusUpdate) ([]StatusResponse, er
 }
 
 func (v *Voidmanager) ListAllUsers() ([]UserInfo, error) {
-	return nil, ErrMangerIsVoid
+	return []UserInfo{}, ErrMangerIsVoid
 }
 
 func (v *Voidmanager) GetUserInfo(UID []byte) (UserInfo, error) {