postgres: tests: improve float64-conversion tests (#80627)

postgres: more tests

pkg/tsdb/grafana-postgresql-datasource/testdata/time_series/convert_to_float64.sql

@@ -1,17 +1,12 @@
 -- SELECT * FROM tbl
--- in timeseries mode, most fields gets converted to float6
+-- in timeseries mode, most fields gets converted to float64
 CREATE TEMPORARY TABLE tbl (
     "time" timestamp with time zone,
     v1 double precision,
     v1nn double precision NOT NULL,
     v2 real,
     v2nn real NOT NULL,
-    c1 text,
-    c1nn text NOT NULL,
-    c2 varchar(10),
-    c2nn varchar(10) NOT NULL,
-    c3 char(10),
-    c3nn char(10) NOT NULL,
+    t text,
     x1 smallint,
     x1nn smallint NOT NULL,
     x2 integer,
@@ -22,9 +17,13 @@ CREATE TEMPORARY TABLE tbl (
 
 INSERT INTO tbl ("time",
 v1, v1nn, v2, v2nn,
-c1, c1nn, c2, c2nn, c3, c3nn,
+t,
 x1, x1nn, x2, x2nn, x3, x3nn) VALUES
 ('2023-12-21 11:30:03 UTC',
 10.1, 11.1, 12.1, 13.1,
-'one', 'two', 'three', 'four', 'five', 'six',
-101, 102, 103, 104, 105, 106);
+'one',
+101, 102, 103, 104, 105, 106),
+('2023-12-21 11:31:03 UTC',
+NULL, 21.1, NULL, 23.1,
+'one',
+NULL, 202, NULL, 204, NULL, 206);