AccessControl: verify metadata map is not null (#43743)

pkg/api/org_users.go

@@ -154,6 +154,8 @@ func (hs *HTTPServer) getOrgUsersHelper(c *models.ReqContext, query *models.GetO
 	if errAC != nil {
 		hs.log.Error("Failed to get access control metadata", "error", errAC)
 
+		return filteredUsers, nil
+	} else if accessControlMetadata == nil {
 		return filteredUsers, nil
 	}