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

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');