Datasource: Fix missing raw SQL query in Query Inspector when query returns zero rows (#67844)

* baldm0mma/issue5799/ add empty frame with all meta data

* baldm0mma/issue5799/ add null test

* write an integration test around query results with empty rows

* baldm0mma/issue5799/ add tests to mssql and postgres

* remove use of apparently reserved keyword in mysql8

* baldm0mma/issue5799

* baldm0mma/issue5799/ update tests

* baldm0mma/issue5799/ update test structs

* baldm0mma/issue5799/ update annotation

---------

Co-authored-by: Michael Mandrus <michael.mandrus@grafana.com>

pkg/tsdb/postgres/postgres_test.go

@@ -1331,6 +1331,50 @@ func TestIntegrationPostgres(t *testing.T) {
 			})
 		})
 	})
+
+	t.Run("Given an empty table", func(t *testing.T) {
+		type emptyObj struct {
+			EmptyKey string
+			EmptyVal int64
+		}
+
+		exists, err := sess.IsTableExist(emptyObj{})
+		require.NoError(t, err)
+		if exists {
+			err := sess.DropTable(emptyObj{})
+			require.NoError(t, err)
+		}
+		err = sess.CreateTable(emptyObj{})
+		require.NoError(t, err)
+
+		t.Run("When no rows are returned, should return an empty frame", func(t *testing.T) {
+			query := &backend.QueryDataRequest{
+				Queries: []backend.DataQuery{
+					{
+						JSON: []byte(`{
+							"rawSql": "SELECT empty_key, empty_val FROM empty_obj",
+							"format": "table"
+						}`),
+						RefID: "A",
+						TimeRange: backend.TimeRange{
+							From: time.Now(),
+							To:   time.Now().Add(1 * time.Minute),
+						},
+					},
+				},
+			}
+
+			resp, err := exe.QueryData(context.Background(), query)
+			require.NoError(t, err)
+			queryResult := resp.Responses["A"]
+
+			frames := queryResult.Frames
+			require.Len(t, frames, 1)
+			require.Equal(t, 0, frames[0].Rows())
+			require.NotNil(t, frames[0].Fields)
+			require.Empty(t, frames[0].Fields)
+		})
+	})
 }
 
 func InitPostgresTestDB(t *testing.T) *xorm.Engine {