postgres: add more tests (to handle zero-rows queries) (#84384)

pkg/tsdb/grafana-postgresql-datasource/testdata/table/no_rows.golden.jsonc

@@ -0,0 +1,36 @@
+//  🌟 This was machine generated.  Do not edit. 🌟
+//  
+//  Frame[0] {
+//      "typeVersion": [
+//          0,
+//          0
+//      ],
+//      "executedQueryString": "SELECT * FROM tbl WHERE false"
+//  }
+//  Name: 
+//  Dimensions: 0 Fields by 0 Rows
+//  +
+//  +
+//  
+//  
+//  🌟 This was machine generated.  Do not edit. 🌟
+{
+  "status": 200,
+  "frames": [
+    {
+      "schema": {
+        "meta": {
+          "typeVersion": [
+            0,
+            0
+          ],
+          "executedQueryString": "SELECT * FROM tbl WHERE false"
+        },
+        "fields": []
+      },
+      "data": {
+        "values": []
+      }
+    }
+  ]
+}

pkg/tsdb/grafana-postgresql-datasource/testdata/table/no_rows.sql

@@ -0,0 +1,12 @@
+-- SELECT * FROM tbl WHERE false
+CREATE TEMPORARY TABLE tbl (
+    "time" timestamp with time zone,
+    v double precision,
+    c text
+);
+
+INSERT INTO tbl ("time", v, c) VALUES
+('2023-12-24 14:30:03 UTC', 10, 'a'),
+('2023-12-24 14:30:03 UTC', 110, 'b'),
+('2023-12-24 14:31:03 UTC', 20, 'a'),
+('2023-12-24 14:31:03 UTC', 120, 'b');