sql expressions: empty results not compatible with alerting

try the following alert setup, it will contain two data source queries and a sql expression: A: - testdata datasource, scenario=csv-content ```csv num, label 15, label1 ``` B: - testdata datasource, scenario=csv-content ```csv num, label 16, label2 ``` C: - sql expression ```sql SELECT A.num,A.label FROM A INNER JOIN B ON A.label=B.label ``` the result contains no rows (this is correct), but the alert evaluation will fail with this error message: ``` invalid format of evaluation results for the alert definition C: unexpected field length: 2 instead of 1 ``` the problem is that the returned dataframe that represents the "no data" has a shape that is slightly different from what alerting expects. we return a Frame with two fields, both have zero items in them. alerting prefers a Frame with no fields at all. this is also the behavior expected by the dataplane specification: https://grafana.github.io/dataplane/contract/#no-data-and-empty