SQL Datasources: Fix variable throwing error if query returns no data (#65937)

* Fix SQL query variable throwing error if query returns no data

* Tests to verify that metricFindQuery returns properly and doesn't throw error

* Fix all codepaths that might throw errors because of undefined backendSrv response

public/app/features/plugins/sql/datasource/SqlDatasource.ts

@@ -164,7 +164,7 @@ export abstract class SqlDatasource extends DataSourceWithBackend<SQLQuery, SQLO
         .pipe(
           map((res: FetchResponse<BackendDataSourceResponse>) => {
             const rsp = toDataQueryResponse(res, queries);
-            return rsp.data[0];
+            return rsp.data[0] ?? { fields: [] };
           })
         )
     );